One could imagine a couple of techniques to unpack the MVar# itself,  and was curious if GHC might employ one of them.

So, really,  unpacking the MVar does not eliminate a layer of indirection,  it just eliminates the need to check a pointer tag (and possibly execute a thunk or follow some redirects if you don't have a pointer to an MVar#).    I think this is what I was ultimately after.

Best,
Leon