I play on Experimental Branch [0.98.20] (don't know if that matters) and you can powerlevel Thievery 100% 'safe' + I managed to level 3 chars to 60 in 10 min playtime.
All you have to do is:
- find anyone unconscious
- put him in prison cell you made (can be in town building you bought)
- strip him of all items then put on him item that has lowest chance to steal (body armor / 2h weapon)
- Wait till he is awake then just steal item you gave him and put it back on him (if you managed to steal item)
- Repeat till you achieve mastery
Easiest change would be to consider Prisoner like game treat unconscious, make him 100% chance to steal, give no experience and/or each time you get caught stealing you lose reputation to fraction you stole from.