If your account isn't flagged for Heavensward, you will not gain experience upon reaching level 50. But if you account is flagged for Heavensward, then you will gain experience in Heavensward even though you haven't finished the ARR MSQ. I have a character that I was struggling to get through the MSQ in order to access Heavensward and she ended up being level 55 by the time she completed Steps of Faith. I was also finally able to do her level 52 & 54 job quests.