Yea iv also heard its impossible to unlock the PCIE versions.... only AGP unlock.
I tested a XFX 6800 128mb AGP and it unlocked from 12/5 to 16/6 fine. However not every 6800 will unlock without artifacts or whatnot.
FYI, it made virtually no performance difference, maybe a frame here or there, and maybe un-noticable stability..... but if it doesnt unlock, its no big deal, performance doesnt suffer..... You would probably only be more conserned about it unlocking properly if you planed on OverClocking it, witch would then probably show different resaults.