I've had the Fernco on my car for about 4 yrs. now, no cracked lid yet. I've never heard of anyone losing power with the TB or smooth bellows.
I figure, A.) I had the fun(?) of doing it myself. B.) every little bit helps, so do as many 2-5hp mods as poss. and it has to add up.
I say go for it! It's a good way to spend the day! Good time to do the bumpstop mod too, if you haven't yet!