I've got the same Powercolor 6970 that mindtheft mentioned. 940Mhz/1425MHz

Win7, Catalyst 11.5

I get ~360Mh/s with defaults in guiminer/poclbm (guiminer 2011-05-21).

If I use "-v -w64 -f0" or "-v -w128 -f0" I only get about 315Mh/s.

With "-w64 -f0" or "-w128 -f0" I get the same ~360Mh/s as with the defaults, so the "-v" appears to hurt performance for my 6970.