I have HFC and mid pipe, all in 2.5inch and this Fujitsubo axle back, which I think it was a factory upgrade option for JDM cars.
As shown in the attached pic, the piping reduces to 2.25 inch (maybe even less?).
I am thinking of cutting if off and put on a 2.5inch pipe. You can see that the inlets of the muffler are actually 2.5inch.
Do you guys reckon it will increase some power by doing so?