It has to be a law of the universe that if something can go out of sync, it will. Over sizing the motors would have the downside of potentially more damage - I'd guess it would be better for the gantry to sieze up sooner rather than later - slightly less potential to put a bend in two expensive ballscrews. Oh well, I suppose at some point I will be pondering how to realign/re-square the gantry - such are the joys of this ... err ... hobby.
Thanks for fixing the links post BTW.