got some more logging done
copied your grill blanking normal comute to work 7 deg ambient and tried to plot alongside last years comute log, which started much hotter . same journey same speeds etc maybe min difference in journey times
the blanked grills help a lot for the few few minutes especially as its mostly off throttle rolling down hill, then once up to a bit more speed the rate of increase seems to slow so its getting cooling air from remaining rad grill section and air from under tray not being there.
I gotta say I thought the difference was going to be much more noticeable, but in these low ambient conditions not so much.
im sure in summer temps 25-30c it would get up very quick but then probably overheat and I wont be repeating in warmer weather anyway.
here are my grills blanked and not(they are quite a large area)
most noticeable really was the rise of boost temps with my fmic suffering mid journey and kept rising
usually this weather id expect 5-10 deg over maf temp maximum
also on return trip more loaded it certainly smelt hotter in the engine bay so im guessing my turbo was also suffering from loss of cooling airflow over it.
and because it was already pretty hot I checked my stat opening temp again
still opening around 88c and didn't drop much just maintained 87-88c but it still took a bit of reving to get it there once parked up again
also took oil cooler temp as parked up thinking it would be pretty high also , but no it was only about 50c same location .