[RedDevil]

I find the grill block starts to affect FE negatively long before the fans kick in.
The reason is air intake temp.
The Insight has its air intake behind the rads, but it takes in fresh air from a snorkel above the rad.
The grill block means very little air flows to the rad, so when it gets hot the heated air rises up to the snorkel rather than flow through. While a hot air intake can give good benefits, it does not work fine combined with an EGR as the recycled exhaust gas obviously is also quite hot.

On cars that do not have an EGR and work well with a hot air intake, it may well be so that the benefit of a grill block has more to do with raising the intake air temp than with actual lower air resistance...