are you compairing these performances based on standing still in your tests or moving? I've built levels where while moving ( even just changing camera pan angle) the frame rate can drop significatly, soon as you stop movement it jumps back up. This all falls back on what appears into view that needs to be rendered.

vid card can make a differnace too as you noted. the engine will revert back to A5 ( all software rendering mode) either by a not direct x supportive car or by scripting it to change. Some of the newier vid cards just out perform others on hardware rendindering speeds (A6 mode) Like i use a gforce 5200 FX card, always have stuck with NVida since the TNT2 GLFire cards but now they just can't seem to perform like the ATIs lately. I've yet to get my games to get past the 60FPS problem with this card.