Actually I meant +10 on top of the +5 you'd be getting anyway. It's a +5 enhancement bonus... and then 60% of the time, you're adding 2x enhancement bonus on top. So +5 minimum, +15 60% of the time. That's more like +11ish on average? And hit chance can really be taken out of the equation, unless you're comparing a specific weapon's proficiency bonus with another: the hit chance will be the same.
So when comparing this weapon property to another at level 21 with a +5 enhancement bonus on a 2d6 brutal 1 weapon, this weapon property does an average +6 damage higher - unless my math is wrong, which it certainly could be.
In the end, after more reflection, it seems like this is just another very good weapon property and not necessarily a crazy broken one. It's strictly better than vicious (since it has the same crit dice), and particularly good with an item like gauntlets of destruction (effectively they give brutal 1). It's also even more powerful compared to radiant than I thought, since radiant gives an item bonus to damage and this is an untyped bonus. That means you can use it with armbands of power, unlike radiant weapons.
However, it's only at its most powerful when used with a very small set of weapons: axes, heavy blades, and maces that use two dice for damage. Currently, that's falchion, glaive, greatclub (all 2d4s), and the new Dark Sun weapon gouge, which is a 2d6 brutal 1 axe - and clearly tailor-made for this property. Nothing else applies (yet), so I think it's not quite as bad as another bloodclaw.