I had only just rediscovered this class earlier this morning when I was looking through my extended signature and had realized that I had forgotten about it completely. Looking through the class, I remembered all of the stuff I liked about it and the incredibly bad timing (making a tier 3 fix for a class inspired by a tier 2 version of a class while said tier 2 fix was on the first page... not my best planning) that caused it to go almost entirely ignored (or at least that's how things would appear).
I have taken the time to fix formatting and make everything a bit more easily read so it should be a bit more accessible now. If it turns out that a general lack of interest in spellthieves was what caused it to fly by the radar, I will let it die in peace.
Also, because I'd feel bad about posting thing without some mechanical addition, new feats:
Subtle Siphon
You can drain spells from a target without the target ever knowing.
Prerequisites: Mystic Siphon 1d4
Benefits: When you use mystic siphon on a flat-footed creature and deal no damage with the attempt, the target is unaware of the attempt until it next casts a spell. If they attempt to cast a stolen spell in this way, they wasted their action with no effect.
Mystic Replication
You can form copies of spells without stealing them from others.
Prerequisites: Mystic Siphon 5d4
Benefits: When you use mystic siphon or steal spell-like ability, you may choose to merely copy the ability instead of stealing it, gaining all benefits as if you had stolen it without robbing the target of resources.
Disruptive Siphon
You can steal spells even as you cancel their casting.
Prerequisites: Mystic Siphon 1d4, Improved Counterspell
Benefits: Whenever you successfully counterspell a spell with a spell level up to your mystic siphon dice, you may choose to gain that spell as if you had stolen it with mystic siphon.
Mystic Rend
When you steal energy from creatures, you do so anything but peacefully.
Prerequisites: Mystic Siphon 3d4
Benefits: Whenever you use mystic siphon to steal a spell from a target, you may choose to have the target take a -2 penalty to its caster level and to save DCs when casting other spells sharing a school of magic with the spell stolen for 1 minute.