Only difference comes in loading times, there is no speed impact to rendering.

The speedup from moving from sprites to polygons is the fact that (please correct me if I'm wrong) Acknex uses quads for sprites which render slower (for whatever reason) than two triangles. Also the engine is 3D based, so the 2D stuff isn't as optimized.

I also concur on using 2^ bitmaps (32,64,128,etc) so the engine doesn't have to fill it in. I do also recommend merging same type of tiles (= less texel fill passes).