When I want a blaster caster, I turn to the Geomancer with Divine Metamagic. At 10th-lvl he's casting 3rd-lvl spells as a 10th-lvl sorcerer, WITHOUT acane failure chances (full-plate and tower shield, here I come!) and using 11 extra levels of metamagic each day on his 3rd-lvl spells, with the option to spontaneously dump all of his lovely spell slots into healing if the need arises (44 spell levels worth!).
Levels: Cleric (3)/ Sorcerer (4)/ Geomancer (3).
Feats: Divine Metamagic, Practiced Spellcaster (Sorcerer), Widen Spell, Explosive Spell, Extra Turning.
My favorite is a widened, explosive fireball, for 10d6 fire damage +1d6 damage for every 10' thrown, up to a maximum of 24 extra bludgeoning damage for someone caught at the epicenter of this monstrosity. You end up with an 80'-wide crispy crater in the midst of a crowd of enemies, ringed with a layer of mowed-down foes. You can do this TWICE DAILY.
No, metamagic isn't underpowered if you do it right