Update:
This week have have driven a lot of miles (about 400) and the grill block has worked great!
From my testing I see a .5-.8 MPG gain with the grates closed. I measure this by closing the grates and watching my instant MPG change. After about 30 of these tests that is the range I see.
Warm up time is also nicely helped. I am now up to temp in 2.5-3 miles. This is about 1 mile faster than with just the upper grill blocked.
I need to mod it though so that I can open only one grate if desired. This morning it was 45F out and I only needed the grill open for 50% of the time (2 miles open 2 miles closed).
Any ideas other than wiring up a second 6 pin switch
?