That's like saying higher horsepower doesn't mean an engine is more powerful.
It is. It's just that not all the power can be transferred to the wheels.
HD2900XT usually get higher Gflops because it have higher shading power. And having higher shader power does not mean that the video card is more powerful because there are other things like texture fill rate
if anything this 8700 is just a steping stone for the 9800's, they probably come out at the same time as the 8700's. look at the 6800gs it camne out around the same time as the 7800gtx.
I'm still holding out on buying my new GPU until November. First to see if they do come out with the 9800s and second to see how the 8700s rate and if it's worth it to pay the extra for an 8800GTS over the new 8700s.