Are you getting linear improvement when going from stock clocks to those 600/900 clocks (test after every 10MHz jump on GPU and make sure the score goes up)? it seems to me your card might be "throttling" there.

Your card at 600/900 = GT4 of 41.8FPS
Mine at 526/803 = GT4 of 44FPS
And that is a very GPU dependant test.

I did have the D3D settings set to 'high performance' but still it shouldn't make such a huge difference..