Regarding diffs - I have rebuilt my diffs ONLY for normal rebuilds and ONLY at fairly normal intervals (actually less than for my GT4 which leaked diff oil and I could never get them sealed well and I didn't want to use grease - never trust the stuff). I've never blown a diff in my NTC3 and have had ONLY breaks that were quite in common with those I've had with my GT4.
Regarding fuel spray - ask Tosselini what happened at the 2001 Norrca Nats. He didn't have his receiver in a balloon or any other protection and it got fuel in it during a refil. The car went 3/4 lap and then went nuts, crossing the track and ending up full throttle against the straightway wall. Before I could get to it (it was about 20-30 feet from me) another car broadsided it and pretty much destroyed it.
So - if you put your receiver in a balloon or some other "receiver condom" you should be fine. In my old GT4 I had to. In my NTC3, I don't have any wires hanging out that I have to zip tie, no receiver exposed to fuel, etc. Is it a mandatory thing? No, it's just something I like.
Regarding exhausts - While there may not be much "choice" - what more choice does one need when Richey himself comes out and says that the RD Logics pipe is as close to perfect as possible. Shoot - I'm still using the original, single chamber pipe with very good results. But, hey, it IS a factor - there's only about 8-10 pipes you can choose from out there. . .
RC10L2.5W - RC12.4 - RCNTC3(bmi) - TC4 (modded) - B44.2 - plus rent-a-rides! :D