The best advice I got was from Eddie Weeks.
http://www.rcuniverse.com/forum/fb.asp?m=5017504
I was setting up a P-180 in an unproven bypass/pipe combination.
I checked the free air exhaust temps, then installed it and checked to see that I was pulling similar temps. The Tam pipe was spot on (Tam knows pipes, period)
If you get a lower temp when installed, you constrict the exit diameter of the pipe to regain the temp (and power). Lower temp is of course easier on the turbine, his theory is that too large of a pipe exit diameter was actually creating low pressure directly aft of the turbine nozzle, therefore making it work less hard (and less thrust)
Constrict the exhaust and the temps come right back, as does the thrust.
For those looking ahead in the chapter, you can figure a cool product would be a controllable nozzle, I had one on my old Viper DF, I have not seen a real one on a turbine but no doubt some German is working on one!!
I think Wren came up with an expanding pipe that increased thust so I don't know how there work jives with what Eddie said. YMMV