They're not completely aluminum. They're an aluminum alloy. EGT probe should be installed pre-turbo of course. Romps above 1300 are "ok" in short bursts. 1250 is "ok" as well, but I wouldn't run it at that temp for miles.
FWIW, I've had a few diesels (6.2, 6.5, 6.6 Duramax, 5.9 cummins) all...