I sold my t5s recently for some CPI in frames. I have never ran CPI pipes before but from what I've read from numerous threads on this site I am really excited to try them. As for t5 vs FMF I have never ran FMF pipes on a banshee, on my old 4 mil short rod banshee I had 5 years ago I had a set of pro circuits on it and I absolutely loved that bike. It ripped hard in the mid range, pulled your arms out and I ride a lot of tight logging roads and trails, gravel pits, etc.
best thing to do is keep an eye out on the for sale section on here and also the face book hq page for a set of FMF and pro circuits. Buy them both and try them out and decided what works best for how your engine is setup. You could probally get both of them for cheaper than a brand new set. Every bike is different l, with numerous things to alter the power curve and ride ability. I have seen a lot of banshees up here in Alaska and all had FMF pipes on them.