My 3" dp and race cat wasn't tight to fit. Far better in every way than the Milltek on my ko3s. Power and torque both increased with the 3" over the 2.5". Every car is different is suppose. But it did feel a lot stronger.
 
Maybe he means 4" tailpipe? like my scorpion tuner 4 cat back
DSC00114.jpg
 
I've got a 3"DP and all runs fine but looking at the space i'm sure you could not fit a 4" in there.
OR if you did get it to fit it would rub and knock on everything all the time.
IMO people should'nt post if they don't know for sure.