It's not only me I swear, this guy from 6 months ago
Methinks it would be overpowered, sounds interesting, but overpowered.
NWN uses 3rd editions rules, which replaced backstab with sneak attack, which does less damage but is easier to pull off multiple times within one battle without the need of going invisible. Thieves gain sneak attack bonuses every other level (I'm not sure on which levels), and the damage is increased by 1D6 each time. The good thing about 3rd edition sneak attack, is you can do it when the enemy doesn't suspect you (not necessarily invisible), flanking, incapacitated etc etc. Also you can only sneak attack with a ranged weapon when you're in 30ft of the target.
In the DnD campaign I used to play a few years back my rogue/fighter wielded a whip dagger (imagine a nimble dagger (more like a dirk) attached the the end of a whip), sneak attacking with that was glorious because I gained flanking bonuses left right and centre.
In my opinion sneak attack is better than backstab, I would love to see a mod (maybe RogueRebalancing) attempt to change backstab to sneak attack. But I have a feeling it's nigh impossible and hard coded. Oh well, bring on the Baldur's Gate mod in the NWN 2 engine!