Yes pretty much. Unless you have instability, like random crashes, changing the psi will not increase computer performance.
If you want more speed consider over clocking your CPU. The 6600 were pretty good back in the day.
It's just that I was getting better frame rates when i was still using my 6770 and after i upgraded to HD 6870, my frame rates somehow dropped. at first i was getting random crashes and shutdowns but after i installed the drivers, it did not crash anymore but like i said i am gettng lower framerates compared to 6770 and that just aint right