So I don't see any reason Y-20's cruising altitude can't be at least as high as the others. Also, tanking doesn't need a fast cruising speed. The Y-20 tanker variant would be a more capable version of the IL-78 tanker, just as the Y-20 AEW/C variant would be a more capable version of the KJ-2000 or the A-50.OTOH the Y-20 is probably not as fuel-efficient as civilian airliners. I would guess that once the C919 is mostly or completely indigenized variants of it will start being produced as military tanker, AEW/C, ASW and JSTARS-type aircraft, just as the US military has turned to civilian airliner variants for its own needs, all of which are versions of the Boeing 707 or 737.
