If you just increase its damages, it will be able to kill creeps itslef and will become a simple AoE tower with "sometimes" a bonus AoE damage, and it makes me think about the Quake tower, with a lower power versus single target...
I think this tower is a support, and its effect is really meant to make chain explosions.
Maybe you could reduce its attack speed to 0.33 and damage to 70~75(maybe a bit less), let the napalm at 40 damage/stack, and increase the AoE (tower and napalm effect). I know its a large rework, but i think it could make this tower much more interesting. it will deal less damage on hit, but place much more napalm effect, making it more significant.