I use a MMM 2200Kv system on 6s and my ESC temps rarely get hot enough for the fan to come on. I had my car geared pretty low and the fan would come on after a few minutes. I'd say you might be geared too low, as opposed to being geared too high. I was running 16/38-40 (can't remember which) and switched to 16/36 and the fan doesn't come on anymore after a good 15-25 minute bashing session at the local BMX track.