Locke, you know, I wonder though whether a pure thief was really necessary. Frankly, thief skills are not often used in the BG series--not to their full extent, as they are in P:T, where at least two dungeons are much easier to traverse if you have a high-level thief. Nothing in BG2 that I've seen in repetitive run-throughs really rewards a thief who goes all out for lockpicking or trap detection. I would say that a great pickpocketer is rewarded by being able to steal high level stuff from merchants, but that's completely undone by the master thievery potions.
Jan can work very well as a pure thief, if you really need that: just give him one of the great enchanted leather armors, forget his mage side, and let him have at it in battle.