A higher-impulse second stage on the Heavy would make sense in payload terms, but have you seen anything from SpaceX indicating that they actually are thinking about doing this? My understanding of their design philosophy is that they prioritise the operational efficiencies of only having one type of engine and one type of propellant to worry about on the pad.

SpaceX's initial money from the Air Force for Raptor involved the possibility of a new Raptor-powered FH upper stage. There's no sign that such a stage is still planned (or was ever seriously considered, really.)

Huh, I'd forgotten about that -- thanks.

Yeah, I doubt it was seriously considered. Probably just didn't want to tell the Air Force what they were really planning to use the Raptor for. Can't say I blame them!