Marr admits that the current beta build is "very old" but he said that Microsoft is trying to get an update out soon. He said: "I think you are going to be impressed by how much work our development team has put into the OS".
I have no complaints about the old beta. It has been extremely stable for me and the performance is great. I don't know what they're doing for the next Beta but I'd guess that they have some more performance improvements as they get better with the architecture. AMD had a very nice gain with SpecInt or something like that recently. No reason to believe that Microsoft wouldn't be able to take advantage of more instruction improvements with K8.