Yeah, new developments are always interesting and worth a try (unless you're a conservative, I guess).
It's abit hard to say 100% for sure, but the Quad 76 IS very wide and offers a very stable platform. I don't think its is more than 76 liters, but for most people it will definitely fel bigger in light wind. But if your unsure and don't want to make a "low volume mistake" again, maybe the 81 is the safer bet. I guess that at 85 kilos also the 81 will be controllable.
Speed: yes, in a pure straight line speed event the Quad will be slower than a single fin and maybe also than a twin. But in most upwind situations you will gain a lot compared to a comparable single fin board and even more relative a twin fin. Once on a plane the Quads track upwind very well and this also lets you bear off and go for fast downwind runs (or lets you get up to the wave quicker). I don't think the speed difference is much of a deal compared to a twin fin board. They are after all also designe to excel on a wave. But a fast single fin will for sure be quicker when pure speed is what matters.