The Archmage's has Empowered 5 and moves 4 squares before exploding. I guess I find it the equivalent of saying that an Axeman's Bronze weapon should give him a greater benefit than a warriors. When in reality, the axemen is simply able to use the stronger Iron Weapons as well.
A brand new mage is only 2-4 (depending on number of nodes) promotions off Empowered 5 move 4 fireballs though. Also, once you have Combat V and Spell Extension 2, that's it. You can't improve the spells any more. Other units get lots of promotions, combat, drill, shock, anti demon stuff, hill/forest defense, and so on. Mages just become more versatile, which is pretty much useless, since you can just build more to cast other spells. I'd really like to see two things:
1) Archmages casting the same spell are inherently better at it. Is at least partly already the case depending how that -spell resist from channeling works. I think the difference should be fairly significant though.
2) More promotions that aren't spells. Mages get what, Combat, Spell extension, and mobility? This means that a fresh Archmage is little different to a 100xp Archmage, because they both have pretty much all the promotions they need, and there's nothing more.
3) Ability to specialise. I'd like to be able to make Archmages that are really good at throwing fireballs, but kinda suck at anything else. Or good at summoning, or whatever. Obviously this means more promotions, so it ties into 2.