My calculation is wrong, cause I used empower spell instead of enhance spell, halving the benefits.
My mistake, sorry!
However, from the D&D FAQ (last release 08/13/02, page 18)
Can you add a metamagic feat more than once to any
particular spell? For example, double Maximize Spell, or
double Extend Spell?
You can apply most metamagic feats more than once. Just
stack up the costs, and remember to apply the additional effects
to the basic spell.
So, the spell
should be:
120 (base intensified)
30d6 (3 x empower)
10d6 energy admixture
120 + 40d6, average 260 hp
In the case of a fireball, twin spell
= enhance spell (10d6 = 10d6).
In the case of a cone of cold, twin spell
> enhance spell (15d6 > 10d6).
So, if you follow the rule "apply metamagic to the base spell", enhance spell is quite weak. It's a feat I'd never take.
It's discussed in another thread, but intensify spell isn't the best metamagic feat around too.
An example (fireball + various metamagic + 1 improved metamagic):
10d6 (base)
30d6 (6 empower)
40d6, average 140 hp , spell level IX
compare to
120 (base intensified), spell level IX
You need a
lot of improved metamagic to reduce the level modifier of intensify, but with 3 i.m. twin spell add 1 level
10d6
60d6 (6 twin)
70d6, average 245 hp
With 6(!) i.m.
120 (base intensified)
50d6 (5 twin)
120 + 50d6, average 295 hp, spell level IX
3 more i.m to get only 50 hp more!
I hope my calculations are correct this time.