My throttle servo had about 10 minutes of run time before it went. I have done the same thing you did. Put a JR 8711 on steering and moved the steering to throttle. So far it's been ok with about 4 tanks through it.
A friend of mines throttle servo was broken out of the box. So again he put a 8711 in on steering and moved steering to throttle. Then that throttle servo went again. He then put a JR 9100S and it has been fine for many races and run time.
These where both on a 8ight-T 2.0 RTR. Horizon Hobby is taking all the servos back and sending new ones back. The servo's are cheap and most likely would go again but at least there are replacing them.