Some people have been seeing better performance in Win8 than even in Win7 (which has been working very well for most people). It's going to be difficult to tell before people start using the final revision with officially certified Win8 ASIO drivers for whatever audio interface someone is using. Most people don't seem to understand that when Microsoft talks about acceptable latency they're talking about WASAPI etc... they don't have any control over ASIO which is really the only thing musicians will care about when using Ableton Live anyway. Just like gamers are always searching for better framerates, musicians are always looking for smaller buffers and latencies. I run my Echo Mia MIDI soundcard at 256 samples even though I can get away with 128 most of the time. It still gives me small enough latencies.
I would just stick with Win7 64bit for now. It's really the best OS Microsoft has ever made (unless Win8 turns out to be better, who knows)