Yeah Davies playing inside centre could actually work. If you had Hook outside him, Scott Williams, or even Bishop, who has been playing exceptionally this season imo. We just need more creativity than Roberts provides. Sure he is good defensively, but you need more than that if you want to win the tight matches (the ones we have been losing all to often recently).
Personally, at this point I would go with a Scott Williams and J Davies partnership at centre, mainly because they play together week-in, week-out. They both have flair and are both pretty physical.
I'm sorry to Roberts but when he gets the ball in open space these days I just know he's going to do the wrong thing. For example, when Blues played Munster the other day, he had a clean break down the wing, from which a simple pass to Owen Williams on the touch line would have resulted in a try. I think any of Beck, Williams, Davies, Hook or Bishop would have put that away, but Roberts delayed the pass for too long (something he is guilty of too often, he seems to become almost selfish in certain situations) and eventually threw it into touch when he knew he was going to be tackled. I just think he has a lot to work on before he gets back to anywhere near the form he showed in 2009, during the Lions tour.