I just did this. (Thanks for the pointer Gordon)
Fuel Line Extension / Stumble Fix Mod - Subaru Impreza WRX STI Forums: IWSTI.com

Common symptom is a hesitation anywhere between 2400 and 3500 apparently. It does recommend you have the map adjusted accordingly as it will now run a bit rich. My 08 STI is due back for a remap soon anyway but initial test drive says it fixed it.