Yeah, that's probably a better investment. Any Bull's Strength spells used by the casters could always be placed on Siobhan, Avril, or Mandla.
I guess Warren could use a +1 shortbow, to help out just a bit with ranged combat. So I'll add that in. I'm assuming he just sells his old bow for half market price. Bought him a Cure Serious potion as well.
I'm about to update Warren's sheet. The summary:
+1 level of rogue, +3.5 HP, +1 BAB, +1 Fortitude, +1 Will, +12 skill points, +1d6 Sneak Attack, Trap Sense +1, +1 Dexterity (resulted in +1 to Dex mod, for +1 to Initiative, Reflex, AC, touch AC, ranged attack bonus, finesse attack bonus, Shadow Blade damage bonus, and Dex-based skill checks); spent skill points for +3 Disable Device, +1 Hide, +1 Move Silently, +3 Search, +4 Spot; and adding Warren's share of loot, minus the cost of replacing his normal bow with a +1 shortbow and buying a CSW potion