SL allows one to cast a spell faster, by risking ESFs... Also, a spell class decreases as a caster's level increases, so it's more a matter of level if your spellcasters "often" have such a problem. Oh, wait, RMX? How different is it from RM2/C? Isn't it supposed to be pretty similar?
Anyway, based on the ESF idea, if RMX has spell fumbles at casting time, you can increase the fumble range, depending on how faster a spellcaster decides to cast his spell (SL suggests roughly +25% if one round faster, +50% if two rounds faster).