If the dev post on it pulls through, they are actually going to simplify and streamline the spell damage routine. They will change it so base damage now *does* affect the +SD damage cap, and also so that the crit multiplier simply multiplies the non-crit +SD damage instead of raising the cap.
I believe that Xanoth is correct about how the crit damage multiplier works. The only other way I could think of it working would be to actually increase the base damage when a spell crits, but that would require a second set of calculations and they are trying to streamline the calcs. This way is best for us - then it gives extra multiplication on *all* non-crit damage, but base, all modifiers and +SD damage as well. Which is how crit should have worked from the start, anyway
Edit: Incidentally, this will both increase the effectiveness of base damage mods, and increase the value of +SD for all spells that are non already +SD capped...