When you lash down a piece of foam you are compressing the air pockets within the foam. So unless you leave the foam uncompressed along the shank, I don't think you would notice any difference in buoyancy of the fly.

I have tried tying stimulators with a foam wrapped abdomen, and pulling tight on the foam to achieve a nice smooth body was enough to eliminate the air pockets and the flies floated worse than traditionally dubbed stimis.

Pulling a strip of foam over the back (like the elk hair on a Humpy) would aid in buoyancy.

Hope this helps,
Adam