We've just dug up a copy or two of Beta Forceware. Here's a quick look at the performance over the existing WHQL driver, the 169.25. Test bench specifications, and RivaTuner shots are below:

Futuremark 3DMark06

CPU overheads have gone down with the beta drivers, yet 3D performance (particularly SM3.0/HDR) has taken a hit from switching to beta. Apparently there's some work to be done over at Santa Clara, for DX9.0C.