I'd stay, if I were him.
Fitness aside he has everything it takes to become one of League's top earners.
I know there's the carrot of international RU money, but I'm unsure of how certain that is for him. Whilst I don't doubt his talent, I think a lot of people are largely basing a prediction he'll take to RU easily on his similarities to Jason Robinson. Now, it was the best part of a decade since he moved and rugby union, especially internationals, are a lot more professional.
Whilst I'm sure Kyle will rip it up at club level week in-week out, its likely he'll be playing wing, full back or perhaps outside centre. When it comes to international level, the opposition are just going to see a really little guy, and pummel him out of the game - and let's face it, Kyle isn't exactly one of those small but tough players who can take the punishment.
If he fails to make the grade at international level, its going to be a pretty uninspiring career playing only club Rugby Union, when he could've been the best in Rugby League.