Vmem mod is not really worth it. 3850's gain much more from GPU Mhz than Mem Mhz. That's why 3850's clocked at 775/2000 perform almost identically to 3870's clocked at 775/2400. Going from 900 to 1050 on my memory gives me just 1-2% fps increase.
My 8800GTS G92 works the other way. GPU increases make little difference because the memory bandwidth is the bottleneck.