Not to burst your bubble about the up-pipe, but did you read into this?
Originally Posted by JSCSpeed
06-07 WRX OWNERS READ: On the 06-07 WRX there are boost spike or creep issues in all cases with the use of an aftermarket downpipe or turboback. This is because the 2.5L engine pushes so much air. Even with the internal wastegate fully open not enough air can exit to stop spinning the turbo, the result is boost creep or boost spike. With the 06-07 WRX, unlike the STi, you cannot fix the problem by just porting the stock turbo wastegate, since the problem has to do with the actual size of the turbo. We suggest porting the stock turbo wastegate AND getting an engine management system to help fix the issue like a UTEC or AccessPORT. With the added costs this will entail, 06-07 WRX owners may want to consider waiting to get an aftermarket downpipe or turboback after they have upgraded there turbochargers or they may want to consider getting a cat-back exhaust only.
Are you planning to upgrade the turbo later on down the line?