Saeviomagy said:
The problem is that picking up almost any other pair of PrCs for epic would serve you far better.
I don't know about that. IMHO, the easiest way to think about MT from a balance perspective is as a standard +1 caster level PrC (e.g., loremaster, mystic wanderer, etc.) with a very steep entry requirement (forced multiclassing) and a "special ability" of 1 level of cleric spellcasting per PrC level. Thus, you get 10 levels of cleric spellcasting as opposed to, say, a few loremaster secrets or some nifty elemental focus or summoning abilities. That 10 levels of spellcasting would be quite overpowered as a class ability if it weren't for the fact that you lost three caster levels to get them. At epic levels, OTOH, you're giving up one epic bonus feat every three levels (in comparison to the wizard and cleric) in exchange for three caster levels of cleric. That's not bad compared to what most caster PrCs get, which is either a 1/3 level bonus feat progression and NO abilities or some relatively minor abilities and a slower bonus feat progression.
A better comparison would be a Wiz3/Clr3/MT17/Acm4 (CL 27) to a Wiz5/PrC17/Acm5 (CL 27), with "PrC" presumably being something like loremaster. (I'm reluctant to bring up incantatrix because NOTHING looks good by comparison to that PrC, and thus it is, after all, overpowered.) The MT takes three Spell Power high arcana to boost his wizard caster level to par with a straight wizard, and has one left over (probably for arcane reach, which can be used with all of the MT's spells, including (urk)
harm). So what do we have now?
Wiz/PrC:
1) 1 bonus feat (Wiz5)
2) Four extra archmage high arcana
3) 10 levels of PrC abilities (some loremaster secrets, perhaps)
4) 1 epic bonus feat or extended epic PrC abilities (unlikely)
MT:
1) Domain abilities
2) 20 levels of cleric spellcasting
I dunno. Are 10 levels of PrC abilities, 1 bonus feat, 1 epic bonus feat, and four archmage high arcana worth 20 levels of cleric casting? From a raw power perspective, probably, but from a versatility perspective, I'd say that an MT would be the best thing going.
Keep in mind that the best tack for an MT, if you houserule a +1/+1 progression, would be to switch over to an epic caster PrC (arcane lord from Dragon 297 is one option; cosmic descryer, elven high mage, or something else might work too) immediately after hitting MT17. That way, you get epic bonus feats, nifty new PrC abilities, and scale just like the single-classed casters do from CL 27 on.