I read that you need to remove them before shuting down the motor as the heat is trapped for longer and cooks the oil in the turbs, i think it was the horsepower in a box website.
I done the $2 cooling mod that was featured in HPI and found the temp drop in the engine bay components was noticably cooler with my self calibrating hand.
I used about 6 washers per bolt on the bonnet hinge to get a lift at the rear of my bonnet of about 10mm, i also removed the rubber seal strip and used some plugs that hold the bonnet sound deadining mat on to fix the plastic bizzo that covers over where the wipers go through.
It cost more than $2 though as i had to by slightly longer bolts to, all up about $10 at Bunnings.
DISCLAIMER, I also wrapped that Flashing tape shit around my Intake and I/C piping, heat reflective tape with a tar backing, DONT DO IT, it took ages to get the sticky shit looking stuff off and it was a waste of about $50.