Non-epic weapons can't have an enhancement bonus of more than +5. It's not entirely clear if this limitation applies to the improved enhancement bonus provided by bane, but consider that if you allow enhancement bonuses to exceed +5 on a non-epic weapon, it can bypass epic damage reduction by RAW.
That's problematic, especially in combination with the Artificer class, which can add something like the Bane enchantment on the fly. You could house-rule that you need a "true, epic" weapon and not just +6 enhancement bonus to bypass epic damage reduction, which I think is closer to the real spirit, but allowing artificers (and anything else which can add bane enchantments) to bypass epic damage reduction willy-nilly isn't very balanced. The ability to add the customized bane enchantment is good enough as-is, without bypassing epic DR.
I'd suggest either not allowing bane to improve enhancement bonuses across the +5 boundary (you could argue this is RAW, even if twisted logic), or special-casing bane; and not considering it an enhancement bonus or not considering the improved enhancement bonus for purposes other than to hit and damage.