oh IIS... that's not ideal. apache can use content type deflate filters which do the gzip on the fly but IIS... I am sure somebody figured it out. still, consider hosting on a CDN at least your js and css if you fail with the rest.
i think the best tool for optimising site speed by doing a series of updates/checks is the YSlow plugin for firebug by yahoo. i can't recommend it enough - it has really helped me optimise some heavy sites. it also has a meaningful performance measure as a yahoo speed index' - assigns a grade that's comparable to other sites etc.