Not 30%, I exagerrated a bit. But I had about 20% fps drop in few games in Win7 DX9 mode, comparing to XP, on this config:
Athlon II x4 640, 4 Gb of DDR-3 1333 mhz ram, video Radeon HD 5770 1Gb, sound Creative X-Fi Elite Pro, HDD Seagate Barracuda 1.5 Tb.
Here's also few more tests...