I don't think it will unbalance things much. As one progresses in spell levels one gets access to, more or less, more powerful versions of the spells he has at lower levels. First there's a spell that cures 1 hit, later one that cures 2, and all the way to heal which cures a unit of all hits, or the different types of damage dealing spells that range from 1 hit to 3 + rout. However, it is rare that a realm spell will completely reverse the outcome of a battle, like a Wrath of God or some such could. However, this are 8th, 9th level spells which no player-regent is meant to have anyway. Only the elves, the Chambelain, a master of the Wizard Conclave in the Imperial City or the holiest of Haelyn's priests are meant to possess such power (and the awnies of course).
I remember how I was drooling to get 4th level spells so I could cast conversion which basically counts as an automatic contest+rule (stealing a temple level from your opponent), but since I started the game by having this done to me, I had to rationalize IC why the use of such a spell would be abominable (since it was done to me of course
) so I figured that, since Haelyn inherited the legacy of Anduiras and Anduiras was immune to enchantment/charm, gaining followers by conversion is a sin in the eyes of Haelyn and even when I got 4th level spells I never learned this one.
You'll find lots of spells that act in such a way, by making a particular action easier or doable in a different way, but no spells below 6th level will have spectacular and miraculous results that would seriously unbalance things.
As for those that do in fact possess such spells (Aelies, Gorgon...) I consider that such characters are powerful and experienced for a reason, and that they know that for each powerful being there are at least 2-3 others who can gang up on him/her if necessary. Even the Gorgon can be laid low if he becomes too big a threat. Tossing spells that raise volcanos and bring about an ice age is a big show of power (and demanding on resources) but it could bring about your downfall.
Thus, I believe that players can play with lower levels without worrying that "but they can kill entire armies with a single spell" will be an everyday occurence, or even that it will happen at all. NPC spellcasters also have domains to run, random events to respond to, have to spend RP/GB to do diplomacy and generally maintain their domains. Someone like the Gorgon will probably have enough reserves to blast a portion of an invading army, but he won't have stockpiles of nukes to destroy the world. If he wanted to do it he would have done it by now or been dealth with by other powerful beings.
And in any case, a temple regent has a lot of other possibilities but realm spells. I've cast a realm spell maybe 3-4 times in the entire 12 turns I played HA and I was always hard pressed for actions to do everything I had to do. You can contest, do diplomacy, adventure...
Same for wizards. IMO, a great wizard is the one not flaunting his power or expending it unnecessarily. Mystery is the key to a wizards power. Otherwise he becomes a stage magician.
