Ian, I reckon a largish slalom board with a 8.4 m cammed sail and you can plane in around 8 knots. If you looked at the gear at Burrum this year, this is the gear that had everyone planning in around 10 knots for three days. Probably the most fun you can have on windsurfing kits in these conditions IMOH. For me thats an iSonic 122, but theres lost of different brands that will do the same job.
Wide tails in general plane earlier, the Gemini does well in light winds. I was stoked at how much fun a tandem is, gybing them is fun too once you get it sorted. I took these pics Aus' Day and we had an 8.3 on the back and 7.4 on the front, got it upto 23 knots that day. Heres one way to gybe em, if you smack your mate on the front you have to shout him a beer