The grenades as they are now are rather badly bugged. Often they have no effect even when an enemy fails each and every save. The only one I've noticed that really works is the sonic grenade. So.. I suggest making the grenades simply cast basic NWN spells.

Frag Grenade: Fireball (level 5)
Ion Grenade: Scintillating Sphere (level 5)
Smoke Grenade: Darkness (level 3)
Thermal Detonator: Hellball

It would also take demolitions out of the picture for determining grenade effectiveness, which I've never thought made much sense.

Basic nwn spells?
Horrable idea.
The custom scripting allows for additional effects, what they really need is a damage boost, and without demo helping grenades it really has little use as det packs are easy to see with tab and can just be shot in most cases rendering them useless.
Also those spells won't solve your problem with people making saves as
10+5=15 dc 15 is pretty low and most people have the feat so you take no damage if you make the save.
Also, Hellball, hell no, hell ball is ridiculously op, and if you give an item hell ball it will just be stupid since it can be spammed, belive me I played a server where we had potions of hell ball floating around, they were irritating.