I also have a Borla. Borla manufactures the Mopar Performance header for the 4.0. I have to say, however, if you have a pre-91 with a Puegeot, you may or may not have to have the crossover pipe modified. I went round and round with Borla with this issue, and what they finally and reluctantly concluded was there must be some tolerance issues with the BA-10 tranny/bell housing setup. It hit the bell housing and wasn't lined up with the cat, because the bell housing was in the way. Took it to a custom shop until it fit. So, be advised. I have heard nothing but GOOD from everyone else however. I would have to say I felt a difference in low end and mid-RPMs.
Also, if you have a pre-91, EGR tube fit perfectly, but O2 sensor wiring had to be lengthened.