In my opinion, and an opinion that is widely shared, if you dare to go over 99 without taking the few weeks to re-write all the game mechanics to fit such, (Sides adding 3rd classes) there is no possible balance. It becomes a matter of which enemies are ohko'd, can't hit because of flee, PD is thrown out of the window, a lot of features become moot, status need to be re-written, elemental property charts need to be lowered. Damage percents become incremential and need to be fixed.. when you heighten base stats, base level and job level a lot of other things need to be brought 'down' not 'up' percent wise. Then it starts to get.. OVD.
I agree though, it is possible with the current mechanics to get around 150 or so with a balance (sort of imba) but when you put 3rd classes into the picture it becomes a kafuffle, not worth it. They arn't completed, nobody should even be touching them yet.