Phong Shading to replace lighting

I am heavily optimizing my app.

We are dealing with pretty big maps and I would like to reduce the tricount some more.

I still need at least some decent lighting.

It would to reduce all the walls in the scene to just two triangles but then the lighting would suck so I thought to use a Phong Fragment Shader.

Would the use of a shader program nullify the effect of the reduced tricount, are there other shader programs for lightings that you would suggest?

Yeah you can use a per-pixel shader, or a static lightmap for old PCs.