Just want to give some friendly information regarding the factory intake tube on 2008 and up WRX's. On the 3 that I have done work on, (all of which had under 40k miles) I have found that the flexible rubber on the turbo side of the tube had deteriorated and was very, very soft. I see two problems with that,
1: There is unmetered air making it past the MAF
2: It's so soft I believe it could be squeezing shut under boost, especially if you're running higher boost levels or a bigger turbo.

So definitely replace the tube when doing a turbo swap/replacement (also a lot easier to do it with the turbo out of the way), I actually recommend doing it whether you're stock or not just to get rid of this weak point.

Just wanted to give you all my .02 on this.