I think they handle sail power on the wave very well. At least as well as an EVO in the bottom turn and in the top turn actually better. With EVOs, you have to pay some attention to how to set up the top turn to engage properly and not bounce or slide and this gets difficult when you are really powered up. With the Quads, the top board will stick to the top turn no matter what. So if you're just reasonably balanced it will work great.
Regarding Q81 or Q76 I can say that I have both (as well as 71 and 66) and I do a fair bit of light wind too. But I'm gonna keep the 76 because I feel its just a better fit to my weight. So at least for me the 76 feels perfect even in light winds (and then I don't even have to consider the high wind end since I have smaller boards for that). Even with the 81 in my quiver I havent taken it out for some time now even in the lightest of days. A set of 16cm front fins will give the 76 even more drive for the lightest days and biggest sails.
But if you still feel you want the extra "pop" and floatier feel of the 81 it will still build a good quiver with the EVO 70. You'll be well into the sweet spot of the E70 before the Q81 shows signs of being to big. And if you're a tinkerer you might even trick your old EVO 70 up with a quad fin setup. The shape is practically the same as the Quad 71.