The niceness of the textures has pretty much squat of diddly to do with using the power of the console. Replace the textures with nice highres ones and you won't be consuming any more GPU cycles. Certainly won't be affecting the use of polygons which the OP refers to.
Any figures people come up with are going to be totally arbitary. You can't tell looking at a game how much it's using resources. For all you know everything's running at 100% and they've just made a severe hash of the optimization. Likewise, if you rate a game at 25% of resources, how can you quantify the improvement using 100%? Because it looks nicer? If you compare XMen Legends II on the PS2 with Champions of Norrath on the same, both the same gamestyle and viewpoint. CON has a high, fairly solid framerate with full screen supersampling and intense particle effects with complex lighting and shadows. XMen Legends II has pretty much none of that with a dire framerate, one of the worst I've personally witnessed. But I can't say that XMen Legends II is only using say 25% of the PS2's power. I don't know how much sitting idle the EE and GS are doing. It could be a poor implementation, or it could be a complex rendering system they use. I couldn't place an arbitary figure based on looks as to how much of a piece of hardware's capability is being used, and I doubt anyone who knows the complexities involved would bother trying either.