I usually run between 22 and 24 pinion (depending on track size) with the 87 spur in my 410 on 6.5 and its fine. I work on the basis that if I can hold my thumb on the motor for at least 3 seconds after a qualifying run, then its OK.
Aggressive throttle action, lots of speedo timing/boost and ESC throttle profile can all contribute towards heat as well - try taming all 3 and see if it improves matters.
I have no experience of the motor you are running but it doesn't sound like its over geared (IMO).
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