I drove my Fiero for about two hours at 110 km/h, and the y-pipe, turbine housing, and discharge pipe all had a faint red glow. It was a dark night; I don't think the glow would be visible in daytime. I don't have an EGT probe on my car.
So I think that some amount of heat is normal with sustained highway driving.
I don't know if it's appropriate to use calculated catalyst temperature as a proxy for EGT. Do you want to know if you're going to overheat exhaust valves?
[This message has been edited by pmbrunelle (edited 07-03-2020).]