Maybe missed his question there. He's already at 50, and wanting to know if he'll still be able to pick up enough exp by the end of HW to hit 60 (presumably mostly through MSQ) if he waits to buy the expansion.

Will you miss out on exp if you continue before buying the expansion? Sure. But I don't really see why that would prevent you from hitting 60 by the end. Also worth considering that when you buy the expansion, it now includes Stormblood, which means the level cap is raised to 70. So, whether or not you need to grind a few extra dungeons, at least you won't hit the cap for a while (and probably will have less issue transitioning to SB).