Honestly, I don't feel like a miner until I hit level 55. What makes you a Miner is your advantage, be it increased speed or increased drops, over other people when you mine. Not being able to make use of this advantage until you're mastered just makes you feel like a glorified/weaker version of every other profession. It's like giving alchemists the ability to make potions at level 55 instead of 30 and saying "we won't let you actually be an alchemist until you master it. For now, you're just a miner until you hit 55". To this effect, I, as a Miner, would mine exactly the same way and exactly the same speed as an Alchemist would all the way from 1-54, with maybe 1 occasional diamond every 1000 blocks mined. The way it should work, in my opinion, is that as you level you acquire new skills over time, which help to define your profession until you ultimately master it, gaining this new, awesome skill that completes your profession. Just giving you one, unique class defining skill at 55 does not, in my opinion, make you feel unique.
I would suggest that you drastically buff the drop chances of extra diamonds (around 15%-20% drop chance at 55) , and add in a chance to spawn gold ore every 5-10% as well. From there, dropping Excavate down to level 40 or 45 would help make you feel more like a miner, instead of just a generic profession, since you'll be defining your class more as you get closer to mastering. Then, finally, allow you to sell/buy ingots and ore at 55. Adding new abilities would also be nice; the abilities that miners get at the moment is a little underwhelming, imho.