yeah good stuff...
so wetsuits: definitely buoyant, it would float all day. Neoprene is designed to trap air in order to reduce weight. Having that will certainly elevate buoyancy.
Is more foam better when in a thickie suit? Yes! Definitely! I think there are two major reasons to do this. One, every stroke is harder in a winter suit, so more volume (within reason) is going to limit your fatigue rate and will increase the length of your session potentially. I know from surfing winters in Maine that my window of peak endurance is far shortened and thus a low volume board becomes harder to use earlier in my sessions. The second reason is obvious, and that is weight. In a 6/5 or 5/4 with boots and gloves you are definitely adding between 10 and 15lbs. So for me I go from 175 to 190...that's a lot!!! therefore a little extra volume is better for paddling and performance.