RE: Acceptable temp for fiberglass around pipe?
I'm no pro on this... but it depends on how you have things ducted inside.
If you have a full bypass... then the heat is contained, and the exhaust thrust should draw cool air in, around the tubes.
If you have it wide ope inside... and a single wall pipe... you should still have the pipe set up, so that it can still create a venturi, and draw air in. To do that... you should have the inner pipe, inside the outer. OR... if you have a single wall pipe... you should use a cup at the rear. ( it also helps draw a vacuum) And then finally... the exit of the pipe should be just inside the rear of the model. That will pull the hot air out.
Every model is different... but the one model I have, that I thought would be a problem, turns out to run very cool. (tight install, and a single wall pipe) The top of the fuse is barely warm after a flight.
I'm sure guys with more experience can answer for that specific air frame.