HS625MG was what I was using for my Throttle Servo. I always use KO for my Steering. Right now I have both KO 2143 doing throttle and steering.
Friend had a 5645 and it died the first time out. Sent it back and got it fixed and three races later it died again. Agian he was using it for a throttle servo. Another guy I know was using the 5925 and had good luck with it...until he got a lot of nitro fuel on it and it died... all it did was make a loud whine sound. I have had my KO steering servo swamped by Fuel becasue the guy pitting it was not paying attention and it did give some problems but all I did was open it up and dried it off and it is fine now.
I know a guy that runs Hitec in his electric car and it does fine except for a loud whine coming from it but he says that is kind of normal and at least you can hear that his car is on