Would you want an APS-C camera with an RF mount?

BirdShooter7 wrote:

As far as I understand it, there isn’t a size/weight benefit from smaller image circle when it comes to long focal lengths like a 150-600.

Of course there is. You have to talk about equivalence. 
So if you want a 150-600 FF equivalent on APS-C then you are talking about making a 100-400 lens. Last I checked the latter is smaller than the former. Additionally if you keep optical formula the same you can degree the lens element diameters, and thus the lens barrel and internal mechanical parts smaller as well. So you get a wight and size reduction like that even if the lens stated the same length. 
But as implied lens design isn't so straight forward, not everything is linear. Like apertures will also determine geometry.

Look at the EF-M 55-200. That is like having a 75-300 (FF equivalent). Loo up FF versions in that kind of FL range and you can see they are huge in comparison.

Of course there is no free lunch. And it is hard to compare crop lenses vs FF considering that we rarely get equal parameters such as apertures. You just don't have many crop sensor f2.8 zoom lenses compared to what you find on FF for example. If you want to keep DOF options the same you get a bigger aperture required on crop, which results in similarly sized lenses etc. 
But generally yes. You can get smaller and lighter lenses for a smaller image circle simply because you can reduce the diameter of the lenses assuming the aperture allows it (f2 is f2 regardless of the format).