Unless he meant underbonnet temps of 99 deg F, 55 deg C, this would reduce power but by nowhere near that amount.
I can't see it would be coolant temp as he would have to get into water system to meansure it, but 55C is SERIOUSLY hot and I doubt very much if it was this, unless it was a very hot day and his RR is not temp controlled ?, in which case not worth using.
tabetha