Shorter mast = earlier planing?

That's a good question and one that I have pondering myself. I think the less extension the stiffer the overall situation, but I would be interested to hear the groups thoughts.

No doubt less extension is stiffer BUT the sail needs shape to work efficiently
My feeling is shorter mast with longer extension will give a better shape when pumped in light conditions, of course sail still needs to be rigged with a tight leech

I have been discussing the idea with many people, and I beleive there's no short answer to this question. From a buddy that has been competiting at national level, using both mast extenders at bottom and top of mast has been effective for him as he is a very light guy (sub 70kg).
Concerning wave sails, the shape is induced by design from the shape of the panels, the battens, luff curve, and fabric layout. I believe that brand make a design for an average rider, probably around 85kg. So the adjustement you can make on your sail profile are :
Luff tension. When bending more the mast, it tends to match the luff curvature thus flatening the sail. Of course you have to adjust your boom length accordingly.
Boom tension also has some effect, and some sail are even designed to be trimmed this way.
Battens can be adjusted, first by triming tension, but some riders even trim their profil to adjust the position of the curve. And then there's the mast rigidity, which affects the way the sail takes its shape under pressure and opens up in the gust. When you have a wave sail rigged up, it is rather flat without pressure. To visualyze how your sail will takes its shape and open up on top, you need to curve the mast sideway.
So usually shorter mast are less rigid (IMCS goes down with length). That's true up to a point : depending on brand, mast shorter than 310 don't have lower IMCS. That's to compensate for smaller sail lower pressure.
Also, kids mast have lower IMCS, and that's another solution for light weight riders (often used by women).
If using a shorter mast than recommended, along with a longer extention, you also have to consider other side effects : the curvature of the mast is set higher, thus this tends to give a hard top effect. And the boom will be set lower regarding to the renforced mast section. You can compensate by using top mast extention, but this king have to be DIY. You can also mix top and lower section of mast (it is even standard methods for Ezzy)

Now why do you need a softer mast ?
I tend to favor these for top end, gusty wind with small sails (sub 3.5m) really require the pressure release valve of the sail twist to trigger at once espacially if you're not a top rider.
As for lower end, it's another issue. From my personal experience, I feel that mast responsiveness and weigth is more effective on pumping efficiency. So in short more carbon is better. Note though that I always keep some loose leech : I really don't like my sails with tight leech, but I suspect that really depends on your sail type.

I'd suggest you made your own experimentation, one setting at a time, after having thinking out how and why you want to affect your sail shape.

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TomDW said..
non-cambered sails obviously do have a much flatter profile with the battens coming in front of the mast unless filled by enough wind.

Well, that's exactly why I think softer mast is an interesting way to use a wave sail for foiling. Plus having discussed the idea with proficient riders, it seems that for the same reason lighter riders gain with softer mast even for fin sailing. But there are other curvature concerns (thus top mast extension rather or along longer bottom mast extension).
But back to foiling, I've been making many trials since I took foiling back in 2018. And though we now have dedicated foiling sail for slalom and racing, there is no consensus regarding small wave sail usage.
I've tried out the tight leech concept as it was commonly thought as the way to go. I've even tried out a 2 batten sail : S2maui Dualist. It is a down the line wave riding sails tight leech design, intended to provide instant power for take of and easy to neutralize in side off. My idea was that it would provide the required 'grunt' to pump up, and then be very light once flying. And it is indeed exactly what I got, but the wind range feels so narrow it is very uncomfortable at my gusty home spot. I tried it out with shorter/softer mast (340 instead of the recommended 370) along with some very long (48cm) bottom mast extension, but the result felt miserable. I've not tried a top mast extension yet though, nor tried out to mix top and bottom mast section.
Now my favorite sail for foiling is an old days Naish Chopper 'M' size I rig on a 340 (with base extension) instead of a 370 as recommended. I've even tried it out on a 310, but the curvature of the mast is too offset upward then.
I don't think there's a universal answer to your question regarding smaller/softer mast being simply better. But sure it does have a real and effective action on your sail behavior. Just be aware that a shorter mast not only change IMCS, it also shift upward the curvature if you're only using a longer base extension. Also, from what I experienced (and I insist on the fact that it is only limited experience), the effect is more noticeable on top end rather than on low end.

I'm not sure an extension is stiffer than the mast. Remember we're comparing the bottom of the mast not the top flexy part. If you were to have a mast made from the bottom part of the mast all the way , and compare it to an extension as long as the mast complete with holes , the extension would be much weaker and flexier. Add to that fact, any sloppiness will produce a kink in the mast to extension join. Even a one mm size difference between the top of the mast extension and the inside of the mast will produce more mast curve than what the bottom of the mast or extension could ever bend. Especially when using more extension thus having a smaller point to point creating a much bigger kink. So I believe , using a longer extension is producing more mast curve on those points alone.