I find it lacking the pathfinder feel. Almost all pathfinder classes have some kind of customization - special abilities you choose from a list, be it Sorcerer bloodlines, Witch hexes, Rogue tricks, etc. This conversion lacks it, imho.
Shadowcaster was always a fun class, but more for its mood than power. It gets a very small amount of spells and even though at higher levels he can cast them more often - campaings are usually at lower to mid levels.
I suggest maybe something like... ummm... "Initiations" that he would gain similary to witch hexes.
Some examples of such initiations:
Unseen casting - Shadowcaster may use a Stealth check to hide the fact he's casting a spell. He can't use it in bright light.
Dark magic - Umbral Spell, Shadow Grasp metamagic feats do not increase spell level
Shadow flow - using your own sustaining shadows ability you may extend its effect on 1/5 levels creature within 15 ft, however, the exposure of the shadow's power on untrained individuals causes them to get a penalty -1 to all physical checks.
Unspoken mysteries - gain an ability to talk with shadows as per "speak with animals" spell. 1 min/level. Each use must be at least 1 min.
Umbral chill - you gain cold resistance 5. At 5th level it increases to 7. At 10th level it increases to 12 and you gain fire resistance 5. At 15th level the resistances increase respectively to 15 and 10.
Seeker in the dark - you gain perception bonus in shadows and darkness +5. You also gain an ability to track other's shadows. To use that ability you must either have something of that person or had touched that person's shadow. You cannot track it if it does not have a shadow.
Conceal presence - you gain Hide in plain sight.
Power of the Hidden - when casting a spell while stealthed you gain a bonus to overcome spell resistance of your enemies equal to +1/4 levels. While stealthed, you also gain spell resistance equal to 5 + 1/every level of shadowcaster.
