freejung - 8:40 pm on Jan 13, 2011 (gmt 0)
I don't know, my experience has been similar to that of seoArt. I made some major improvements a few months back, such as image optimization, that dramatically decreased my total payload. I also improved rendering time, number of requests, caching, and various other factors. These improvements had a noticeable impact on both the performance I personally observe and on the performance reported by various tools.
However, there was little impact on my WMT performance data, which is claimed to be very accurate. There was some impact, but not much. I'm skeptical. I think there's a reason why that section of WMT is in beta.