Yeah it's funny how noone ever seems to mention having probs getting the old bolts off. Of the downpipe installs I've been involved with (3), one of the top bolts seemed to be crossthreaded from the factory on 2 of the installs, and I twisted one of the studs off the bottom on my car. They would of been easy if we didn't have to deal with that.
As far as not taking the intercooler off, I don't think it's possible w/ the STi. Even w/ the wrx, I'd be afraid of beating it up pretty bad. Plus it seems like it'd be too hard to get to the bottom studs.