Then again, even if you make them 200 Astral Pearl national summons, the Sidereal will be seriously out of whack out-of-the-box SCs. I mean, insanely high Astral, sacred, luck, twist fate, quickness, magic resistance, attack and defense skills to put pretty much anything else to shame. At only twice the cost of a Doom Horror, that's quite a deal. At the cost of a Doom Horror plus one casting of Gift of Reason, that ain't half bad either.
Hence my original suggestion to forget game balance altogether.
If you want them to be true, full-power exalted without restricting them to special scenarios or throwing balance out the window, I suppose you could make them special heroes, available only at ultrarare special sites(Like the Gray Knights), or only in specific rare special events, or even combinations of events and sites/units.*
Of course, finding one of these sites or getting one of these ultrarare events would still be, in plain terms, an outrageous stroke of luck which wins you the game. A few Dragonblooded, the least smackalicious of the Exalted, could make short work of, say, a fully equipped Heliophagus. Therefore, players are likely to See this sort of workaround as more of an arbitrary, aggravating, random cheat mode sorta thing than an enjoyable gameplay feature.
Erm. Suggestion retracted.
All things considered, I am being kind of pessimistic, though. I'm sure the Exalted can ultimately be balanced, as long as you don't make them a nation like any other. However, unless you drop the Sidereal altogether, I do suspect you're going to have some major headaches preventing The Ultimate SC... then again, the Sidereal aren't supposed to actually work for any nation or fight anything, are they? What if you just made them a truly horrendous special event, like Bogus on PCPs? It's pretty much common knowledge that if the Sidereal ever go to war, it'll be the end of all creation, but one or two doing unpleasant things for unfathomable reasons is definitely acceptable.
With the Sidereal out of the way, everything else CAN be balanced, albeit tricksily. I mean, Lunars would make truly spectacular fighters and stealthy units(Plus a dollop of Nature, and a dash of Astral), but have very little SC potential.
Of course, once you actually start thinking about giving the Exalted a proper niche, rather than making them just like Pangaea, Abysia, Ermor, Marignon, and possibly Caelum combined, you open up a whole new can of worms. The 17 nations in Dom 2 are truly spectacularly balanced, and each has a unique range of strengths and weaknesses. The human nations overlap a little, but are ultimately very, very different, when you get down to it.
What niche could a balanced Exalted nation possibly fill? What disadvantages and, Pretenders help us all, advantages would they have, and how would you preserve the rock-paper-scissors equilibrium?
*I know this can't presently be modded, but at least it's already been done by the devs themselves. We know for a fact that special events can be linked to specific units/sites. Now all we have to do is invest as much effort in modding Dom 2 as the devs invested in creating it in the first place. I motivate, you mod.
Of course, if this actually can be done, the Exalted might make an interesting desperate Last resort for a losing nation. You spend insane amounts of gold and resources on a "Generic Prophecy" immobile, stealthy, noncombatant unit, and every turn, you have a 2% chance of an Exalted appearing to actually fulfill the generic prophecy in your darkest hour.