NP are big into promoting their boom cut out concept and many other brands have adopted it. Is it a gimmick or benefit????.
I use Pryde sails and the set up seems to work well but it's hard to quantify the level of benefit from this feature alone.
I had a 9m NP RS1 sitting in the garage because I couldn't justify the cost of buying a big carbon boom for it's occasional use. So I went to a sail maker and had the boom cut out, in by 15cm, like my newer Pryde sails.
Did it make a difference?? Well my existing carbon boom now fits nicely. 
I used the sail the other day and it felt very different (could be partly my imagination as I have only used the sail twice before on a new board).
The sail felt a little gutless (its probably lost 0.1m under the boom) and I couldn't drive the board upwind. After a bit of fiddling with boom height and mast track I found a setting that worked quite well. However I felt that the sail still didn't seem to be as powerful as before the mod, but I probably need to find the new correct outhaul setting. Overall I was surprised that I needed significant re tuning of the set up.
So what do the experts think about boom cutouts and it's effect on sail performance.
Yes I think that it will depower the sail considerably as it will allow the leech to twist a bit more and lot lower down. The new NPs are designed for it, but to do it to an old sail could render it useless.
It would good if you could post a few pics, rigged of course... From the head and foot to highlight the twist and across the leading edge to see the depth vs height profile.
Would it ever be possible to have a batten protruding from the end of boom instead of the cutout? I'm not sure how you'd attach the boom though.
edit: just realised I am far from a sail expert, just asking is all
I've used Aerotech VMG's & Naish stealths with the cutouts- I'm now back on longer boomed slalom sails. I do like the handling the shorter boom gives however i've found its easier to oversheet and yes it is very easy to kill the power... with the adjustable outhaul it requires about half as much tension to have the same effect as with a longer boomed sail.
How much your sailmaker charge for doing the cutout btw?
I'll take some Pics next time it's rigged, no need today as it was 20-30 knots. 
The clew cut out cost $55 at the local sail maker. Much cheaper than a new carbon boom.
Some pics of my modified sail. I think the RS1 was designed as a high wind formula sail and it's certainly got some range. I sailed it all afternoon in a solid 20 knots gusting to 25 and it was very well behaved. Maybe the cut out helped.
Hey guys,
Well, it would interesting to know why the cut out.
Sails with cut out are,
Ka
Gaastra
Neil Pryde
North to name a few
Sail without are
Ezzy
Maui
Goya
Loft
I guess there is a mixed bag out there which might indicated that it could be only a concept rather them real benefit.
Regards
Adriatic Blu
Gaastra Vapor don't have cut out.
Loft Blade do.
North don't but play around with booms lengths alot.
I've looked at a few sails since theis post was done, there has to be a crossover batten for it to work like that RS-1. Looking at the Gaastra's it would look kind of odd unless you did it in the style of the Aerotech VMG's which did suck a bit especially if boom near max as you have to extend it further to make it past the batten. Waiting4wind possibly could have gone a bit bigger, I think most brands take about 20cm off vs old but at least you've tried it and shown it works.
Adriatic Blu I think without a cutout you get a more even twist, which is not always a good thing- some race sails used to be designed so batten above the boom fell away more to induce twist cutouts naturally do this. It doesn't matter how good a sail is, if the boom isn't stiff it lets the draft move etc. Boom cutouts ensure maximum results from your boom. The downside is they change the shape of the sail slightly, but with smallish 20cm cutouts it shouldn't really be a problem.
I think those sails with crossover batten it is only a matter of time before they introduce some form of cutout.
I should have had some photos taken on the water to see how the sail looks under load. I had it (or me) maxed out it in 25 knot gusts which would have shown the twist nicely. Next time.......
I was also thinking about measuring the amount of boom flex just to see how much deflection there is on my 6.7 vs 7.8 vs 9m.
I thought about making a measuring device using two pieces of string loosely tied, so one slides on the other under tension. I rig the sail so it just touches the boom about 3/4 of the way back. So I'd tie one end on the boom and tape the other on the sail where it touches the boom. Then give the sail a workout and see how much longer the rope is.
I was reading this thread only because I was filling in time @ work then something grabbed my attention! Waiting4wind, would it be possible to see more photos of the lady holding the rig? I would contribute something more constructive but I don't have a clew!
Having had a bit more time on my modified sail I thought I'd give you an update. Had a light wind session yesterday and discovered a huge bottom end after a bit of time spent tuning.
I mentioned in my earlier post that the sail felt a little gutless and I thought that this may have been as a result of the boom cut out. Using just a little less downhaul made a big difference and also made the sail a lot more tunable on the water with an adjustable outhaul. This sail was designed around a really stiff mast (NP CK100), but the X6 I'm using is considerably softer, so I suspect its easy to kill the power with too much DH.
Overall I'm now quite impressed with the modified sail and I would recommend the upgrade. Having used it in 20 knots and still manageable and yesterday planing comfortably in 10 knots (Isonic 111) I can't complain about the range. 
The sails with the new boom cut out have less power but they feel nicer to sail like depower better in gusts and you can jibe better. And you can rig a smaller boom on ya race sail. they save you money as well cause ya only need one boom for most of ya sails now with having a cut out on your bigger sail. you could use the same boom for ya wave sails ect...
Think thats a cut out on the NP, they will make the boom cut out even bigger in the next 5 years there going to look more like a hang glider in the future. All brands will follow.
cams will get bigger and luffs will get wider and in the future they will never fill with water and the cut out will be even bigger.
we will be able to sail 6.5s in 30 knots with a smile on our face.
I put the question re boom cut outs recently to Barry Spanier on the Mauisails forum here: http://www.mauisails.com/forum/viewtopic.php?pid=11355#p11355 (post #4); Barry's response is in post #5.
chris
A cutout might be a good/useful idea, but the Neil Pryde description appears to be just marketing:http://www.neilpryde.com/sails-2009/sail-technology/dynamic-compact-clew.html
The example shows the clew/leech section twisting off by an amount approaching the radius of the deepest part of the foil.... how? Specifically, there is far more skin tension in the first 30% of sail (including lots of extra cloth to fill that curve), than with the trailing edge - how can the back end of a rod-batten twist by that much?
I love the little "see it in action" animation. That warp tool in photoshop is great hey!!!!!
