I've got a couple of questions, if you don't mind.
Which vehicle are you doing the grill block on, the truck or the car?
When I did my grill block, I blocked the whole thing, 100% and then added opening's as the temperature gauge indicated a need. I ended up running with 2 openings about 2"*4", to get sufficient air to keep the engine at normal temperatures. I used a sheet of cardboard, which was admittedly pretty darn ugly.
In consideration of relative values, I had perhaps 5% of my grill space open. In contrast, you have about 33%, it sounds like. I am wondering if the difference your seeing isn't affected pretty substantially by the comparatively high amount of area you still have open.
Their are a couple of considerations, from my perspective. I did the test in about March. I have not run the test in the Summer. I have considered, but probably will not try to do a grill block on a trip planned across the desert, in the Summer. In all, a grill block does make a difference, but it is probably 5% or so. In contrast, I don't want to cook any of my engine, because I do not like rebuilding blown up engines.