I haven't been able to find anything regarding padding vs. no padding. The few results of tests I have found regarding drag on BCs shows that the difference really is negligible provided you're talking about minor differences. Clearly if you've got some sort of massive BC stuffed with padding to the point it looks like its going to burst, you're going to create some drag. But I'm talking about minimal padding on the shoulders/hips compared to no padding on the shoulders/hips.
Unless you're in some sort of underwater race I don't really see where these minimal reductions in drag would come into play. It should also be noted that generally speaking, when you see a comment on a website about reducing drag with a BC, its usually made by someone trying to sell something. That always makes me a bit suspicious.
Not that there's anything wrong with wanting to be a bit more streamlined and not have 39 pieces of flare attached to your BC. But there are much better reasons, I think, to do it than to cut down on drag.