Any ideas on how many cu ft added would guarantee the boat floats with a full load of fuel and 4 people?
It all depends on the weight of the boat. You are looking for neutral buoyancy. I'm sure someone will chime in with some numbers shortly. As far as pros/cons there was just a discussion on this
here, it starts at the third post and goes on to the next page.
Also, keep in mind that the foam will not keep the boat from rolling over, it will (should) keep it from going to the bottom. Also, in order to foam the entire hull you will be cutting a lot of holes to access all of the areas for pouring. Personally, I would leave it alone but it is your boat and your call.
Good luck!