i have AP2 (07) model, but i'm going to get an AP1 rear bumper.
Will the J racing diffuser fit without modification to the (bumper, frame, or the diffuser)?

 

I recently swapped an bumpers out (put an ap1 on my ap2) and the biggest problem is that there are two brackets that bolt to the bottom of the AP2 bumper that hang way lower than the ap1 brackets so you'll need to get them changed or removed. They're welded to the chassis

 

The rest is a bolt on when you swap your bumper? M not worried with the 2 bar hanging as long everything bolt on quickly..

 

J's racing Diffuser has a bracket that needs to be mounted there....for the AP1 is shorter, and as you know the AP2 is a bit longer...the bracket won't work correctly and hang way lower in the back.

You might need bend it to the height of the AP1 bracket then drill a whole so you can mount that bracket correctly.