Yes, that's correct. The RuntimeRep story generally allows for such polymorphism in the future, but it's not implemented yet. One concern is that doing this would require RuntimeRep to look like

The problem here is that, in the vastly common case of kind *, this requires an extra indirection. I actually implemented this, and indeed the performance of GHC decreased. Given that we have no way, yet, to take advantage of the kind of polymorphism you seek, I flattened the structure of RuntimeRep in order to get a modest but consistent performance boost.

So I'm, personally, very open to this direction of travel, but we have to work out both how the polymorphism should work and how to implement this without degrading performance.

Richard