Maybe you need to review your configurations? Assuming you have an adequate CPU and your are running at a fairly high resolution, you should be seeing noticable performance gains in SH4.
Unless you don't feel that doubling your frame rate is worth the time, you might want to give my suggestions a try.
EDIT: just to qualify this a bit more... my 8800gts cards are both of the 640MB variety. Folks with the 320MB cards may not see quite so dramatic an improvement.