Aftermarket header to fit Pro Circuit T4 slipon?

My newish XRR has a T4 slipon fitted to the stock header right now. I'd like to update the header. I've been told the FMF header will fit and allow me to make it to the T4 i have on now? Anyone ever done this or able to confirm? What about the XRS only header? I really don't want to purchase a whole new system just to get a new header.

Ask the manufacturers if their pipes will fit a stock muffler since the T-4 slip is made to fit the stock pipe (and won't fit the T-4 header system.)

Clear as mud?