I did this with black aluminium the day the car was collected from the stealers.
I've done over 3k miles now, round town and high speed motorway cruise - I've had no issues with temperature. As was already said, road use will be fine, track use, maybe not....The car takes ages to get the oil temp up to normal so it can't be blocking too much airflow
I opted for the easy route lower down and put the mesh outside the factory plastic, fastened on with very small cable ties. Very easy to remove when it becomes damaged.
The kidney grills were yanked out (don't follow the ytube videos of removing the bumper, just give 'em a damn good tug evenly and they just pop out) and the mesh formed into the shape and cable tied inside this time.
I didn't want leaves etc getting through the kidneys and falling down inside the bumper area so the mesh is tight up against the back of the kidney.
I even did the two grills by the fog lights, drivers side has (what I believe is) the oil cooler in there, the passenger side is a blank but had to be done to match...
Overall I'm happy enough with the result, though I may well go down the plastic route if these don't last well.
eBay, £7.90 ish. Can't go wrong at that even if it needs doing every year
