Performance Testing Guidance for Web Applications is now available on MSDN. I am still going through the version I downloaded a few weeks ago, now I'll probabyl read the whole thing on-line - I find that somehow having the table of contents on the left-hand side help keep me focused. Perhaps it's because information is so dense? (there is little you could take out that wouldn't remove something of value)
A lot of the information available on the guide is applicable to Astoria projects. I'm tempted to say 'most' rather than 'a lot', but I'm only half-way through the guide at the moment.
In any case, the world is a better place with better-performing applications, so go ahead and read up all you want!