As far as when to shift for max performance, It's really up to you, but I shift @6k, and then at 5700, and 5.5k-6k depending for max accel.
As for when to shift for best mileage, It just depends. but remember - the lower your RPM's - the less gas you're using. I try to drive around with my tach between 2k and 2.5k. I'll even go into fifth for the long stretch of road when I'm only going 55mph.
When to shift is not as important as how hard you accel. If you go into boost, and then shift it at 3k all the time, you're gonna see worse mileage than if you just let it go all the way up into 4k. I would say, drive it easy, and shift between 2500 and 3500. IMO, of course.