Well I cannot see Farrell being an international fly half going forward. Like his father he has the skills set of a rugby league loose forward. Apart from his goal kicking the rest of his game is pretty unexceptional. I would still view him as a centre. Once Twelvetrees gets regular rugby at Gloucester he will be there or there abouts as well.
I definitely agree about twelvetrees.
Farrell is just too slow for a centre for me though. If he's complimented by a 12 and 13 with genuine running power then I think he can play a role, as he's got a cool head, good defence and is a good distributor.
But at 12 I think we'd struggle to score tries.