I use them on mine in summer. I haven’t done any proper tests to measure temperatures under the bonnet but I do get an impression that they do help in that the fan seems to cut in less frequently. That may be due to better airflow out of the engine bay reducing some back pressure reduction allowing easier airflow through the rad(?). I took them off for the winter. When they go back on I will do some test runs to see if the incoming air to the filter is lower plus a thermometer to get some readings.
I do get comments about the bonnet not being closed!
