Sunder isn't really overpowered in D&D. The use of it almost always entails loss for the user - the loss of some good solid damage, AND the loss of a potentially very expensive weapon that he could sell or make good use of later.
Even bad guys need to pay the rent. Remember that.
It ends up being a tactic for use in fairly desperate situations only.
Disarm is a whole new ball game. Although it still suffers the problems of lost attacks, it's significantly better in that it's a one-shot solution (as opposed to the damage mechanic of sunder), and that it doesn't lose you any money. OTOH, it can be protected against with a locked gauntlet, and it can also be fixed after it occurs (by picking up the weapon).
As to level coming into it, and the debate about hitpoint mechanics etc. Disarm is an opposed attack roll. I think that pretty much says it all. Fighters will succeed about 50% against other fighters of their level, and success is very closely linked to combat experience (ie - levels of full BAB classes). I think that using a hitpoint mechanic would probably be a bad thing (why does my CON help me hold onto a sword??).
I'd agree that disarming is probably a little too easy to accomplish for someone with improved disarm. OTOH, that's an investment of two feats, one of which negatively affects disarming (expertise would affect the disarm roll, so it's either unused, or it makes improved disarm worse). And of course that investment is worth very little when you come upon opponents who are larger than you, have natural weapons, or have locked gauntlets.