[RedDevil]

I suspect your A/C radiator is behind the lower grill opening, so that might get better cooling instead of worse.
Blocking the upper grill means less air in total flows into the engine bay, so the under hood pressure is lower which helps the air flow through the lower grille.

You may want to monitor your air intake temperature though. If it draws air from behind the UGB, it will take air that is hotter than without UGB. That may benefit FE, but can also be detrimental if it causes pinging or if your engine also has an EGR.

I've got a cheap in/out digital thermometer sensor dangling in the intake opening of my Insight. It usually reads a few degrees above front bumper temp, but raises quickly in town. As it just got hot over here I've now removed most of my LGB.