JMM has to stick to lightweight to have any sort of success. There's still great entertaining fights down at 135 against Valero and even Katsidis.
i think he'l do fine at 140 as long as he doesnt get in with the best right away.. hee should probably get a couple tune-ups in before going in there with hatton,campbell, or judah at 140.. maybe fighting maligniggi at 140 or even diaz again wouldnt be such a bad idea for him to adjust to the weight...
I hope JMM will stay at LW and unify the belts at that weight which he is surely capable of in order to seal his legacy as the best Mexican fighter of this era. I think he will still be competitive at Jr WW but the added weight will slow him down and probably lose to the best fighters of the division.