RE: patriot XL,rear ex.
I'm not all that sure you will see much more performance with a FIRE 90L as oppose to a standard side exhaust 90L.
The XL is a big bird..... quite a bit of drag even with the gear up and stuff. It will take more than the extra 400-800 rpm you will see with the full pipe (same prop) to return a measurable speed increase.
Power and design wise, the FIRE and side exhaust 90L are about the same. You can get more power/rpm from the rear exhaust, but this is mainly due to using the big pipe and having it tuned for a more specific rpm target (higher RPM, smaller prop perhaps).
I do know of one customer who has a FIRE90L intended for an XL, but he is using the quite pipe.... mainly intended to meet some field noise requirements. Setting the pipe a bit long, turning higher pitch..... a bit lower rpm (Shooting for 12,500 if I recall... its been a month or so)
You should not have any issues with turbulence or anything from a pipe running down the side. I use to set up pattern ships like that on occation. Drag is not really an issue with the pipe. A bit better than an even trade for the side exhaust muffler. Your decision to run the RE version is just a matter of a trade-off of some more complexity (and cost) for a marginal return.
I will defer any further specifics to anyone who has built a RE version of the patriot.
Bob