That doesn't really make sense....those traits are essentials for test cricket- not necessarily ODI cricket- and he's already shown how he can cope mentally with the longer version of the game. I sorta agree with the fact that he would be a massive liability in the field due to the fast nature of one-day cricket and how every run is absolutely vital, but with regards to temperament and concentration, I'd say he's covered those two very well for an emerging sportsman.
By the way, I know of Joyce, but was just wondering; how does he go about it with the bat? I mean, does he look to score early and impose himself on the opposition, or ease into his innings, etc...