The ability seems powerful but quite possibly balanced.
Comparing it to other abilities, it's worth the same as both Wounding and Bane or both Keen and Holy. So, comparing it other abilities, the +3 equivalent is worth about 55 points of damage and 4.2 points of con/round in the hands of a fighter who is using a specialty weapon against a vulnerable foe. Well, 90-110 points of damage per round or -20 SR or so? Not an obvious choice.
Keen and holy is worth about 50 points of damage per round in the hands of a 20th level full attacking fighter. So, about 50 more points of damage against nearly all foes or -20 SR or so against foes for whom that's relevant? The SR penalty sounds interesting but, again, I don't think it's completely obvious.
If you ignore the fact that enhancement bonusses are limited to +5 and achievable through Greater Magic Weapon, a simple extra +3 enhancement bonus is worth somewhere around an utterly reliable 40 points of damage/round in the hands of a 20th level fighter.
So, I don't think it's obvious that this would always be a better choice than standard +3 abilities.
Furthermore, a lot of creatures have multiple sources of SR. Most high end evil outsiders, for instance, have Unholy Aura at will which provides SR of it's own in addition to their native SR.
However, there is reason for concern since:
1. SR is nearly ubiquitous at high levels. At level 20, it's not a specialized enhancement; it's useful against everything.
2. An archer using this ability could easily reduce any single foe's SR to negligable levels with two hits. And since archers almost always have the ability to make full attacks, and almost always hit, this could easily make SR a non-factor in most of the PCs' battles.
3. Eldritch Manawall Crushing?!? Talk about a mouthful of a name. Enhancements should have simple names like vorpal, or speed. Brilliant Energy is pushing it but Eldritch Manawall Crushing is completely over the top.
So, based on that, I'd be inclined to allow it as a melee-only enhancement at +3 and see how it worked.