Oh for the 415M yeah that's wrong. But as Novum stated 6 TA/TF isn't possible. It's 8 TA/TF and for the GT420 which Neliz linked above it's measured correctly.
That 2% figure maybe happened in a game with no foliage? Cause AAA can cause big drops on my HD 5770. It's less of a hit than my HD 4670 but it can still be quite a difference. I fired up some GPU limited scenes in Mass Effect, and there was definitely a difference. With 8x MSAA I got 60 almost...
GTX 480 is only capable of 30x 32-bit pixels per clock so it actually has less fillrate than both the HD 5870 and HD 5850. Not counting Z and memory bandwidth limitations of course.