I've never felt my cheap-o suspension seatpost robbed me of power either. And while there is a small amount of slop (1/8" maybe) when I grab the saddle and wiggle it, I can't sense it at all when I'm on the bike.
The weight kills me every time I hammer out of the saddle or try to bunnyhop. Mainly due to the height of the weight like Kimmo says. When you're stomping on the pedals swinging your bike to and fro that extra weight up top is unpleasant.