I have the brake dust shields on both my cars and I've not noticed any problems with heat related brake fade. I certainly get lots of chances to try it out on the hills around here too! One thing to note though, there is a definite left and right on the shields. If you install them on the wrong sides or rotate your wheels and forget to change the shields you likely will have a overheating problem.

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LRG
1987 300D Turbo 175K
2006 Toyota Prius, efficent but no soul
1985 300 TDT(130K miles of trouble free motoring)now sold