The 66 seems under-finned to me -- it could actually use bigger fins for riding in higher wind and better upwind angle. The 66 doesn't seem to have too much more low end. . It's more like it just has more buoyancy.
The 66 rides flatter, much more like a raceboard, and the fins do feel a tad on the small side to do the board's size justice. That said, is still a very easy freeride board for it's size, and the tuttle boxes are there to whack in some serious fins if you are wanting to really point. That said racing is very different to fun sailing..
We had a big session last week with a number of riders all playing on the 54, 60 and 66. For the first time we put the centre fin of the 66 in the back of the 60 to make it tri, and the decision was pretty unanimous - 3 fins was faster, more planted, more lively and once a bit of power applied points just as high with more boardspeed. It's possible that 4 fins made it plane slightly earlier, and held more absolute power when you railed it over in big gusts... But when the centre fins come available my advice would be - get one!
Depending on the temp of where you ride I would think your wind ranges given will be full power on a sector - you could get going a LOT earlier, I can get a 60 going in 6-8 knts with a good 10m in 15 degrees.
The V3 is a beautiful board.. we have sold out in the UK until end aug, and secretly I'm glad I don't have to hand mine out for demos. I'm enjoying the lighter wind stuff a lot. The novelty of hacking at full power in 10-15kts is not wearing off...