here I risk probably to oversimplify your problem. If I do so, sorry about it.
When you talk about lighting and 2D sprites, I always think about an overkill. A huge hammer (which costs you in implementation time + rendering time) for a very small nail.
There are plenty of ways to simulate lighting in a very cheap and efficient way.
A typical example is to draw a bitmap with alpha simulating the light point of effect which creates a very good sense of "fake lighting" on isometric games. There are (or better, there were) plenty of commercial games using this trick.
An example of this https://gamedev.stackexchange.com/questions/22159/can-i-achieve-a-torchlight-effect-lighter-area-around-a-light-source-in-a-2d-g
If hypothetically you could use a solution like this, you could achieve outstanding performances at parity of solution since this effect would be much much cheaper than any other lighting calculation.
I hope to have helped in some way.
Cheers Maurizio