I'm guessing it takes a huge amount of time to get even one level when you're that high though.
It's definitely slower, but some skills level faster than others. Block levels REALLY slowly. Illusion and Conjuration level fast. I got bored enough over level 75 that I used an exploit to gain levels fast by speed-leveling Alchemy (fortify restoration loop). But something like One-Handed levels pretty fast once you set it to 15, assuming you use it a lot in fights.
One thing that might make things easier for a Thief is to make Sneak legendary and just lean on powers, gear, and spells while it levels up again. Thanks to all the gear and special spells unlocked from Thieves Guild quests, you should be able to turn invisible, muffle yourself, and so on, making it easier to remain undetected while you level up the skill again (and it doesn't take too long to get to the "Silent Roll" perk again, which is the main one I notice missing once I can't use it...).