If we deem the inverse effects to be no more powerful than the normal effects of Bestow Curse then we would end up with everyone who has access to a 5th level Cleric or 7th level Wizard running around with a permanent +6 (untyped) bonus to all ability scores, +4 (untyped) bonus on attack rolls, saves, ability checks and skill checks, and potentially the 50% chance of taking double the number of actions on each of their turns. As a DM I doubt I would let that fly and I don't think I know anyone who would.

I think it's fairly ludicrous to suggest that those inverse effects are remotely equal in power to Bestow Curse's normal effects. However it would also create a world where Remove Curse and Break Enchantment would be used offensively which I find quite amusing.