mamlin - You raise a good point, however the test is a little tricky I think. To be a fair comparison, I'd need a similar site. I obviously can't just make a copy of my existing site and put it up on a different URL, since Google definitely will detect a content copy and refuse to index the site. I don't have any new sites to deploy right away, otherwise I'd test on one of those.
The issue with a custom sitemap.aspx is that you can't just edit an xml text file and save it as an aspx file, since .NET will try to handle it. You'd need to edit the app. So it'd be easier to just test with XML files if possible.
I guess the ideal thing would be to deploy three 5-7 page test sites, each similar but substantially different content so the Googlebot doesn't think they're the same content, and submit a sitemap.aspx (with default .8 or .5 priorities) for one, sitemap.xml (with same .8 or .5 priorities) for the second, and sitemap.xml (with different priorities) on the third, then track the response time on Googlebot. If both XML files are quickly handled, then it's the extension, but if only #3 is quickly handled, it's the priorities.
This sounds like a lot of troubleshooting work. I'll see if I can find some time.