LOL at the idea casters are so much better that L10 casters would be better alongside L20 non-casters.

Better at what? Spells? ok, the others, by our definition, don't cast spells. Combat can't possibly be meant, is it?

I think the easiest fix of all would be to sheer off the L6-9 spells from the game. Leave them increasing in CL, and they can have the HL slots, useful for extra low level spells (with or without any metamagic feats they have).

I'd let heighten spell applied to summon monster/nature's ally get the critters of the spell heightened to. Probably needs a few other such tweaks.

Call it a low magic world if you like (but that's not really true). One can leave in items with L6-9 spells, on occassion, created by the gawds or other ubers. HL monsters might need some tweaks for un-escapable spells, but not all that much I expect.

At least this way the casters have some HD, HP, saves, skills, feats & CL (that matters vs monsters). Perhaps the WILL still put fighters, rogues & the rest to shame, but I doubt it.