"WHAT MATTERS IS THAT HE PRODUCES."
Well, of course. The same goes for other players who will be at new positions, e.g., Babin, Wong, Pitts... The only thing I disagree with is the claim that Coleman is undersized, which if he is undersized, then about 80% of the FS's in the league are undersized. If the point is now whether player X will produce because he is "out of position," then that's a totally different argument which is more difficult to prove one way or another.
I happen to think he will be as good or better at FS than he was at CB the last two seasons. He has the physical ability to cover from the top -- it's more a matter of what goes on in his head and whether seeing the entire field is confusing to him after spending so many years looking at eyes and numbers. His problems on run support were usually due to getting caught out of position by overpursuing or being drawn inside and losing outside contain.