CUDA 6.0 might need adjusting or tweaking and it's not an official release yet just an RC for people to look at and devs to develop on, there may be issue or adjustments to software needed
Well, an RC is pretty much the finished version and only put out to make sure that there's no major flaw creeping into the release.
There was a preview version that was handed out last year under an early access program (around 2-3 months ago)
I am not very positive that these performance issues will be addressed before general release.
Christian