Thought that might be the case
Few suggestions:-
Turn all the toys off, no boost, no timing, no nothing. Just run the motor as is. Even remove the physical timing on the endbell.
You may even have some limitations available in your ESC to restrict power on start up (current limit IIRC)
Play with transmitter settings. You can either cut off the top 20/30/40% of the throttle (EPA) or change the response, think it's called curve on mine (may be exponential). Means that 50% trigger is only equivalent to 30% throttle, sort of. Blunts the initial kick off the line. You'll still get full speed with exponential/curve, but lose a tiny bit with EPA.
Slower motor (maybe 10.5) then as you get better turn the timing and boost up, you should be able to get almost back to 8.5 performance eventually, then pop your original motor back in and start turning the wick again.
You can probably find second hand or cheap motors for 30 quid. Hobbyking/club/eBay would be good places to start.
If motor temps are ok now, i'd not be doing much on the ratio front, maybe a tooth up/down on the pinion, or 3 each way on the spur. At most.