I have an xforce catback. And, its a 3". Thats right, 3".
I have had it since new in 2005, sounds quite tough at times.
I completely forgot its a 3" and somewhere along the way, years ago, just assumed it was a 3.5" - i recall buying 3.5" but whatever thats 5 years ago. I do however the company it was from (justjap) saying it was direct fit - well, that wasnt true, costing more in hourly labour.
Now its making a tad over 600hp at the wheels, I think its time to go larger, but i have to say for the price they are, they arent a bad buy. Although for anyone reading this and looking to buy one, dont be put off by the brand, mines doing the job....