There are several successful MX riders such as Fred Andrews and Rodney Smith that rarely won a title in MX, but raked in many major offroad titles after their MX careers were over. Ryan Hughes, too, but WORCS is pretty much an MX on a really long track.
I think it is just a matter of having some experience in the woods. The good MXers I've ridden with have been able to figure it out pretty quick, even on technical nasty trails. IMO, with few exceptions, if you are good on a track, you can figure out how to be good offroad.
That being said, just because you can run with a fast woods rider for 5 or 10 minutes does not mean you can keep him in site for longer than that. Shoot, lots of guys can go fast for 5 minutes. The top guys go that speed all day long, and by the end of a 3 hour race they are an hour ahead.