This was something that we discovered at Turbo-Tunes in Greensboro NC. but I was not entirely sure of at the time. Mark, who runs that place, is a Grand-National genious and can build a heck of a turbo'd mustang... but he's not as familiar with the Subaru platform. So we were learning.
But we did some late-night UTEC road tuning and the bang-for-the-buck seemed to come from lean fuel and conservative timing. Timing advances didn't make a lot of difference, but the leaner condidtions were well felt even with the butt-dyno. And the bad knock results were a lot worse when adding timing than when pulling fuel.
I've been working on a 22 psi setup for my STi ( I have a very safe 20 psi map so far to work with) and trying to maintain richer than 10.9 under WOT. But I think I might pull timing and go for a bit leaner setup to see what happens.
Of course the weather is getting super warm in a hurry here to throw that monkey wrench into the works.