Ithaca has to beef up their doubles to handle the higher pressure loads that came out after WWI. You can get away with shooting a few hot loads in a Flues if you are lucky but long term you just going to regret it most likely. Not so much a blowup risk as cracked stock risk and I hate restocking a Flues.

That's why I reload for the few 20 ga Flues I shoot. They are so light that carrying them is very comfortable but a steady diet of factory loads would end their use of quickly.