Novak brushless
I started running the 5.5 motot and GTB speedo in 12th a couple of weeks ago and I've never had a thermal shutdown. Even when I was really overgeared and only getting 7 minutes of runtime it was only about 125 deg on the speedo (no fan) and the motor was even cooler. If you're having a thermal shutdown you're definately overgeared.
From what I've seen so far, you need to gear way down with the 5.5 motor. Gearing down won't necessarily make the car lose top end but it will run a lot cooler and won't dump.
I had mine geared like a 9 turn and was dumping on carpet so it looks like you need to gear even lower (like a 1.30 rollout or less) for 4 cell 12th on carpet.