The method I use is to add in their Spellcraft skill to the roll, making their skill in forming a spell be put into the mix. If I remember right others on this forum have done similar. Also go 'old school' and don't follow the rules in all circumstances. Make adjustments (behind the screen) that you deem suitable based on circumstances in game at the moment. How calm does the character seem? How potentially distracted? Is s/he taking time to get it right? Physically and mentally how tired, sore, wet, hungry, cold etc is the character (take it from an old Marine, those are BIG factors in performance)? If they're ready, rested, or just seriously dedicated and/or pissed off (and working through the 'pain' so to speak) then they SHOULD get a bonus to their casting. Not huge, but enough to maybe put them over the edge. After all, in 'Thieves World' the characters were, and thus your's SHOULD be, tough determined bastards!
Violence doesn't solve everything, but it solves an awful 'lot!