From what I've read the best indicator of how much cooling a radiator will be able to deliver depends on the difference in pressure across it. For instance
Undertrays, Spoilers & Bonnet Vents, Part 1
A search of this forum didn't find any mention of people actually making this sort of measurement before and after grill blocks, and it seems to me that it would be useful information to have. The article describes various pressure sensors which read the difference in pressure between two tubes - the simplest of these is just a transparent U shaped tube half full of water. There are also small solid state sensors which can be read electrically, although I have no idea what these cost. Seems like it would be easier to run a few wires under the hood than a pair of tubes which must not be crushed.