Why would putting dividers in make the lighting more intense? I know what you are asking, but cannot fathom why you are asking it.
Your light is measured as the lighting over the complete volume of water it is illuminating. You can put a divider in every 2". It doesn't matter, the light intensity and the total volume of water remains unchanged....24w over 24g is 1 wpg. 24w over 4x6g is 1 wpg.
A more effective measure is LSI (Lumens per square inch), which measures light intensity over a footprint, and your footprint isn't changing by adding the dividers. The rear "overflow" section, yes, changes footprint... is that where you got the 1.6 wpg from?
The only consideration I can think of is that your dividers are black and not clear. This hurts you, as the light cannot spread properly - meaning you may (will) have lower effective lighting rates versus an un-compartmentalized and equivalently sized aquarium.