To the OP...

Your attempt to do this by manipuating a very clean 3.X system of XP awards is...misguided.

If you want to slow casting progression down, then probably the simplest approach is this:

Your caster level, and the number of levels you can take in any primary (ie 1-9) casting class, is limited to 1/2 your character level. Any class, ability, feat, etc. that would break this stricture is banned.

Quick, simple fix that has the same basic mechanical effect as what you describe, while allowing these now-restricted casters to spend the rest of their XP diversifying into something else that interests them.


I don't know how WELL that fix would work...other posters have already pointed out some of the problems in the concept and ways that even low-level casters are very effective and powerful.