For those who don't know what I'm talking about. Take a standard Fireball, it deals Fire damage.
Energy Substitution(cold) replaces that Fire damage with Cold.
Energy Admixture(cold) duplicates that Fire damage with Cold damage.
Admix is noted as stacking with Sub, even if it's the same element so the above can be combined to deal double Cold damage (the converted fire base + duped amount as cold). The *trick* is to take Energy Substitution twice allowing you to change the element and admix in a different one so your Spell deals two different elements and has both Subtypes allowing it to benefit from all kinds of bonuses.
Along comes the MiC's Ring of Mystic Fire & Ring of Mystic Lightning. For 7,500gp each these Rings come with 3 charges per day and can be expended to add dice to any Spell that deals damage of the same element. It's generally too expensive for single nukes. Longer duration Spells, particularly Persistable ones, are a better usage. Now, Admixture states it adds equal to normal amount which means the Ring bonuses shouldn't be included in the initial double effect, however since the Spell is both types you can add both Rings which are indeed different Magical providing different, yet similar, effects which gets around Stacking issues.
Notably examples of abuse is Fire Shield and Wrath of Flames. Fire Shield hits anyone that attacks you with a melee weapon for 1d6+15 (or 8d6+30, 58 avg) for some nice vengeance. Wrath of Flames on the other hand is a level lower and is offense focused, dealing it's 2d6 (12d6, 42 avg) damage to anyone adjacent to you at the end of your turn and with each melee attack. Of course, while Wrath of Flames is pretty abusive (but not on the scale of dragonfire inspiration or the durd's acid spell) these are not all that helpful to a pure Spellcaster who doesn't plan on out melee'ing a Fighter. Discordant Malediction provides an excellent opportunity to really squeeze those dice in, dealing up to 12d6 every single time you cast a Spell with a Verbal Component, provided you can obtain a means to negate damage dealt to your self or even absorb it (eg be a mechanatrix). Maybe by such means as Spellguard Rings (can't check my self). An unaltered Fireball for instance would deal it's normal 10d6 damage, plus potentially 12d6 just for casting it. Casting Grease both trips foes and you blast them for 12d6 damage. So nice.
This can be done with most of the typical Metamagic tricks. The Spell it's self lacks an area so no Explosive Spell but augments like Fiery Spell would add an additional +12 damage for +1 Spell level while the Fell line from LM would additionally debuff enemies. Because who says you can't have your CC and deal damage too?