I
haven't looked at tire specs, but what you say seems reasonable.
It also makes sense that wheel/tire mass makes little difference at constant speed.
But I'd still get the lighter wheels if I could. The majority of my fuel savings comes from the P&G technique = repeatedly accelerating (and at a fairly brisk rate). In those circumstances, or for driving that's primarily urban, it makes sense to reduce wheel/tire mass. For highway cruising or DWL, not so much.
About the question of RR being solely a mass issue, I wonder how much aerodynamic lift a typical car generates at highway speeds...
(Just kidding.)