I know of about a great many aircraft that have the throttle servo mounted ON the engine in a much tighter cowled environment. No problems, ever.

I do most of my planes throttle servos similar to Altavillan's. Keeps the linkage short and direct. You have to figure, those that run cannisters and tuned pipes under their radio installations direct far more heat to their receivers and rudder servos than what is radiated in open air aft of the engine to that servo.