Aft of the mufflers, I think the temps are a bit lower than 500ยบ or so I seem to recall.
I tested it with a thermocouple back in college for a plastic tailpipe design (yes, you heard right)...and something tells me it was lower. WOT might be up there, but that's not where you're after the noise cancellation anyway. I don't have a thermocouple handy, but your calculations look spot-on!
Cool thread.