I am using the 3 1/2" pipe. It takes a little modifying to make it fit right because it hits the strut tower and wont let it reach the t/b unless you cut it. For this pipe it says you can ONLY use a short pro-m, they say that because they don't want someone to buy the pipe use something else, and have it not work good, which is understandable, but I am using a 94/95 GT MAF in the fender with no problems. There reasoning for not being able to put other MAF's in the fender is because some go crazy when you do, or because its too far away from the t/b. Doesn't make much sense to me but that don't advise doing it in case someone does have a problem they cant come back and blame it on them. Like I said though 94/95 GT MAF in the fender, no problems. As for power increase, cant say I noticed much, but I didnt do it for power, I did it to clean up the bay a little, I did notice it idling a little better though.
Pics of the 3 1/2'