I haven't tried it, but people here whose opinions I respect say that they have layered SG and UPP with no problems.
One reason to do it would be the way UPP is slicker than SG. I prefer how it looks too.
I do find SG to be the more durable product. If you get 4-6 layers of SG on there (I know, that's a lot...) you could probably just spot-clay (very gently) and add UPP for a very, very long time. The SG (let alone the paint underneath it) seems to stay nice (i.e., it doesn't degrade) as long as you keep it clean, and the UPP would be a "sacrificial layer" over top of it.