I'd be careful running UPDP on an my04+. SOme have documented massive amount of detonation. 04+ have a longer closed loop fueling position (this means they run a lean burn longer) due to enviroment issues from the EPA that forced subaru to do this.
By having your car custom tuned, you are able to reduce this dealy and get fuel in there before your start to hit higher boost numbers. THe reason UP DP could make it worse, it because removing the cats/back pressure from the system will allow your turbo to spool faster and could leave you running nearly full boost at 14.7:1 AFR's (that's not good)
Of course everyone's car runs different. SOme people DET on a stock car, some mod it with DP and UP and have no issues at all.
Tuning is a VERY important part of modding cars like these. They're not the same as modding old v8's where you just throw parts at it and it makes more power. It's just a different set up.
Do yourself a favor and get your stage 2 components, and get a protune (some have even picked up more than 15WHP).
well i dont think we have to worry about him running catless being that hes from cali. So he will prob be ok with running a full turbo back with a cat in it. Ive had 2 04s that i ran with just turbo backs till i got an ecutek and a utec and they ran great. But its important to remember that were dealing wit imports and no 2 cars are the same. Some car react differently to some mods. It also depends on where you live as well.