I remember seeing someone post a thread about the difference between the stock gutted up-pipe, and an aftermarket straight up-pipe, and they hit the dyno. They had other mods, but the gutted up-pipe proved to be identical to the aftermarket pipe.
Well, I went to the 1/4 mile. I have a 2005 WRX. I have done only one mod to my WRX. A gutted up-pipe. I bought a used stock gutted up-pipe from someone for $15 on ebay (best $15 I have ever spent).
I took my car to the track in November and in 40 degree weather, I ran 14.607 bone stock.
I got the uppipe in January, and installed it one day with a friend.
I went to opening day yesterday at MIR, and in 50 degree weather, I ran 14.355. Same launching techniques.
I didn't know if anyone else had made a notice of the difference in the up-pipe like this, but for those who had questions about the difference it makes, here's your proof. It's worth it, and it's worth the bloody knuckles and cuts that you'll get from installing it.