Actually, it's 30.0; it was originally 80.0 in Age of Shadows and before, then cut to 50.0 in Samurai Empire, then FURTHER cut to 30.0 for Mondain's Legacy.
Ah, yesh I would go find another NPC (try looking for thief guildmaster NPCs in some Felucca towns, IIRC) to buy training past 30.0.
Also keep in mind the above values for armor; and if your total armor penalty surpasses 42 points, you will ALWAYS fail stealth; so the max, end-game suit you might use for training would be, say, a metal (ring/chain/plate/etc.) tunic, with metal legs, and studded gloves.