The first mod I want to make is to replace the stock exhaust on my V8. I believe there is already a Ford Racing exhaust, but it it safe to assume that current axle back exhausts from Borla or Magnaflow for '13 and '14 mustangs will have the same fit?

 

I don't think so. I saw a YouTube video that shows the pipe connection points are now different for the 2015+.

 

Definitely not for the '15, if that's what you are asking.
The IRS diff doesn't move, so exhaust goes under the inboard hard points of the lower "A" arms.

 

No. The platform and exhaust plumbing on the '05-14 is totally different than the '15.