Your build could use some tweaking. Try this:
Spellthief 1/ Sorc 4/ Unseen Seer 2/ Spellwarp Sniper 5/ Arcane Trickster 8
Spellthief 1 - for the 1st level skill points
Unseen Seer 2 - Advance Learning: Hunters Eye spell from Players Handbook II
Spellwarp Sniper 5 - Ray Mastery is very nice
Feats:
Master Spellthief - Complete Scoundrel
Craven - Champions of Ruin
Invisible Spell - Cityscape
Versatile Spellcaster - Races of the Dragon
Darkstalker - Lords of Madness
Persistent Spell - Complete Arcane
Guided Spell - Dragon #307, Dragon Compendium Volume 1
Arcane Blast - Pathfinder: Advanced Players Guide
Spells:
Snipers Shot - Spell Compendium
Guided Shot - Spell Compendium
the first level with Pathfinder doesn't really matter like in 3.5, in-class skills automatically get a +3 to them if you have ranks in it, but you don't get the x4 at first level, all in all you actually get more in the long run.
I'd also have an extra 2 feats based off of Pathfinder progression, 3 if I decided on Human, although I'm leaning towards Tiefling since, in the Pathfinder version, they don't take the Cha penalties if they have the Infernal or Abyssal bloodlines, but I'm sure you knew that since you actually listed a PF feat from the Advanced Players Guide.
The only thing I really don't like about this build is that...I feel dumb now...with Spellwarp Sniper, it makes the 10th level redundant in Arcane Trickster, but I guess not getting the 9th level of that that late wouldn't really be that huge of a deal other than it's a free action.
...you might be rather godly with this...I'm looking for a major flaw and at 7:24am, I can't find one...
...except for the fact that it only really becomes playable between level 7 and 12 when Spellwarp Sniper is takable
I'm really having some trouble with the spell selection due to the fact that Sorcs are so limited. This might be the department I need the most guidance in.
I don't know if it will help, but this is my current sheet from DnDSheets
http://www.dndsheets.net/view.php?id=8941