Caster Level and the Cleric/Mage

I really like the concept of the multiclass Cleric/Mage as a character who has mastery over both streams of magic. But one thing worries me and is discouraging me from making one: caster level.

A bulk of spells (dispel magic, fireball, skull trap, Abi-Dalzim's horrid wilting, pierce shield, finger of death, etc. etc. etc.) have a caster level modifier. This can effect how much damage a spell deals, or how effective it will be generally.

Is this a big problem for the Cleric/Mage? Am I correct in thinking that, for instance, casting a fireball spell will only count the mage levels (and not the cleric levels) as "caster levels"?

I've heard that all of these caster level effects are capped at level 20 (with the exception of skull trap). If this is true, then the Cleric/Mage wouldn't be so inferior to a pure Mage or Cleric of equivalent experience in ToB. Can anyone confirm this? I could live with having less-powerful spells while levelling up through BG1 and most of BG2, in exchange for having equivalent spell power (and C/M versatility) in the final quarter of the series.

Anyone have any thoughts on this issue?