Here is a sample of why a Mystic Theurge is way over powered as it stands. I made a sample 'vanilla' character with the following Stats using the Paizo 25 point buy for Epic:
Human 20 total levels
S10/D10/C11/I20/W20/CH10
Feats:Empower Spell, Enlarge Spell, Extend Spell, Spell Focus(Evoc), Spell Penetration, Spell Mastery, Widen Spell, Greater Spell Focus (Evoc), Greater Spell Penetration, Heighten Spell,Eschew Materials, Quicken Spell.
I configured him 4 different ways and calculated the number of spells known as well as the number of total spell levels each combination can cast.
I configured him as a level 5 Wizard/Level 5 Cleric/ Level 10 MT
5 WIZ/5 CLER/10 MT
Wizard Spell Level 0 1 2 3 4 5 6 7 8 9
Spells per day 4 4 4 4 4 4 3 2 1
Bonus Spells 2 1 1 1 1
Total per day 4 6 5 5 5 5 3 2 1 0
Base Save DC 15 16 17 18 19 20 21 22 23 24
Evoc Save DC 17 18 19 20 21 22 23 24 25 26
Cleric Spell Level 0 1 2 3 4 5 6 7 8 9
Spells per day 4 4 4 4 4 4 3 2 1
Bonus Spells 2 1 1 1 1
Total per day 4 6 5 5 5 5 3 2 1 0
Base Save DC 15 16 17 18 19 20 21 22 23 24
Evoc Save DC 17 18 19 20 21 22 23 24 25 26
Total
Total Spells Spells
Per Day 8 12 10 10 10 10 6 4 2 0 72 Total Spells
Total Spell
Total Spell Levels
Levels 0 12 20 30 40 50 36 28 16 0 232 Total Spell Levels
Notice only max of 8th Level Spells for both Cleric as well as Wizard which seems to be an issue. So lets take a different approach and using the same stats lets look at going Level 7 Wizard, Level 3 Cleric, and level 10 MT. that would give access to 9th level wizard spells (or you can transpose them and go 7 levels of cleric for same result)
7 WIZ/3 CLER/10 MT
Wizard Spell Level 0 1 2 3 4 5 6 7 8 9
Spells per day 4 4 4 4 4 4 4 3 2 1
Bonus Spells 2 1 1 1 1
Total per day 4 6 5 5 5 5 4 3 2 1
Base Save DC 15 16 17 18 19 20 21 22 23 24
Evoc Save DC 17 18 19 20 21 22 23 24 25 26
Cleric Spell Level 0 1 2 3 4 5 6 7 8 9
Spells per day 4 4 4 4 4 3 2 1
Bonus Spells 2 1 1 1 1
Total per day 4 6 5 5 5 4 2 1 0 0
Base Save DC 15 16 17 18 19 20 21 22 23 24
Evoc Save DC 17 18 19 20 21 22 23 24 25 26
Total
Total Spells Spells
Per Day 8 12 10 10 10 9 6 4 2 1 72 Total Spells
Total Spell
Total Spell Levels
Levels 0 12 20 30 40 45 36 28 16 9 236 Total Spell Levels
The results are very Close, gaining only 4 total Spell Levels but now being able to cast up to 9th level Wizard Spells and 7th Level Cleric Spells. All of this seems in line until we compare it with a 10th level WIZ/10 CLERIC as well as a straight 20th Level Wizard. Lets compare them now using the same information.
10 WIZ/ 10 CLER
Wizard Spell Level 0 1 2 3 4 5 6 7 8 9
Spells per day 4 4 4 3 3 2
Bonus Spells 2 1 1 1 1
Total per day 4 6 5 4 4 3 0 0 0 0
Base Save DC 15 16 17 18 19 20 21 22 23 24
Evoc Save DC 17 18 19 20 21 22 23 24 25 26
Cleric Spell Level 0 1 2 3 4 5 6 7 8 9
Spells per day 4 4 4 3 3 2
Bonus Spells 2 1 1 1 1
Total per day 4 6 5 4 4 3 0 0 0 0
Base Save DC 15 16 17 18 19 20 21 22 23 24
Evoc Save DC 17 18 19 20 21 22 23 24 25 26
Total
Total Spells Spells
Per Day 8 12 10 8 8 6 0 0 0 0 52 Total Spells
Total Spell
Total Spell Levels
Levels 0 12 20 24 32 30 0 0 0 0 118 Total Spell Levels
Notice the 10wiz/10cler can only cast 5th Level Spells in each, loses 20 Spell levels compared to the MT and 114 Total Spell level as well! That is a huge disparity.
Now on to the 20th Level Wizard using the same stats:
20 WIZ
Wizard Spell Level 0 1 2 3 4 5 6 7 8 9
Spells per day 4 4 4 4 4 4 4 4 4 4
Bonus Spells 2 1 1 1 1
Total per day 4 6 5 5 5 5 4 4 4 4
Base Save DC 15 16 17 18 19 20 21 22 23 24
Evoc Save DC 17 18 19 20 21 22 23 24 25 26
Total
Total Spells Spells
Per Day 4 6 5 5 5 5 4 4 4 4 46 Total Spells
Total Spell
Total Spell Levels
Levels 0 6 10 15 20 25 24 28 32 36 196 Total Spell Levels
The Epitome of of spell casters...the wizard at 20th level gets only 46 total spells per day and 196 Total Spell Levels. Compare that to the level 7 WIZ/3 CLER/10 MT and you will see that the MT casts 72 spells per day compared to a wizards 46 or 52 of the 10wiz/10 Cler. The MT also casts 236 Spell levels (zero level still count as 0 as they are unlimited for all examples) compared to 118 for the 10wiz/10 cler and 196 for the straight level 20 Wiz. For Pure Spell casting the Mystic Theurge dwarfs any other core casting in the game. Now the question is what do you gain or lose by going MT?
Losses: Depending on the level of cleric you take you will lose a number of Domain powers. On the wizard side you will lose some of the School Powers if you got that route and some bonus feats (3-4 depending on how many wiz levels you take).
Gains: Massive increase in the sheer volume of spells you can cast is the biggest. Also the Combined Spell abilities abd the Spell Synthesis at 10th Level MT. Being able to cast 2 spells at the same time (one from each class) is a fantastically powerful ability.
So why is this so overpowered? Well let me ask this...after looking at this after I made 4 characters...all middle of the road...what spell caster would NOT choose this presgtige class? IMHO this is still the most overpowered class in the entire game dating back to the 3.5 rules.
Now you can min/max with stats in each example and tweak feats etc...to slightly alter the final character, but to me this is a no brainer when playing a spell caster as you are the most powerful caster in the game bar none. Who would play a loremaster after this? Or really any straight class! The bumping of the Core classes was needed to balance the game, but this goes above and beyond any bumps given to the core spell casters!
Some ideas we had before the Paizo conversion was to alternate class progression (arcane one level and Divine the next) and add more abilities. Another was to only let it be a 5 level Prestige class. Yet another is to have 'dead' levels were you do not progress levels on either class. All I know is that if I wanted to add a broken class to play, I would stick with WOC. I do know that as it stands now, I would not allow this class into my Pathfinder game.
Am I right on this or missing something?