An empowered Blade Barrier is an 8th-level spell, and it has no realistic chance of killing a dragon in 2 rounds. Furthermore, the dragon gets a save against Blade Barrier, and then gets to move away (no damage next round). Yeah... I'll cast the 6th-level spell, it's better.
The Earthquake example wasn't very good. How often do PCs face hordes of weak creatures, making an appropriate challenge? You'd be better off casting Fireball or Fire Storm.
Heal is porobably broken when used by a 600 hp dragon. However, it's not broken when used by a PC, who won't have that kind of hp until epic levels. ELH discussions, BTW, are another thing.
"at such a level" a fighter can do mucho damage.
Really? Without using a smackdown or deliberate looking for loopholes, I've seen a psychic warrior do 80 damage per round. I've yet to see a psychic warrior do 600+ damage per round,
even if using Pssychofeedback with a scythe, Haste, Power Attack, and landing all crits!
Against a fully armored and buffed cleric? That is unlikely.
Clerics tend to have poor touch ACs, even if they do use Prayer, Bane and Shield of Faith.
So, there should be someting In between, a touch spell that does mondo damage.
You consider doing 600+ damage mondo damage?? Isn't 250 damage enough for you? Sheesh... even Power Word Kill doesn't have that kind of power.
But look at it in the wider context--those are the best clerical spells, hands-down, for dealing or healing damage. You put a cap on those and clerics can't stand up to wizards anymore.
See Power Word Kill. You don't need to make a spell overpowered to balance the cleric. The cleric's spells are almost as good, he gets more spells/day, better saving throws, better AC, more hp...
And how is this different, power-wise, from a spell that can instantly restore 1000+ hp?
You have a PC with that many hp? Find me a non-epic monster with that many hp.