Let me clarify... sorry if i was a bit vague. Here is a prime example.. I will use myself.
I have my gearing set to THEE sweet spot for the gear I run, and my track. I run
Fantom Apex 17.5
12.5 HT rotor
maxed end bell timing
31/69
~120 after 8 minutes
The car is SUPER fast for a 17.5. It has the perfect balance of rip in the infield, and pulling speed down the straight. On COUNTLESS occasions, and at multple tracks (albeit similiar sized tracks). I have gone down to 30/69, and tried 32/69. Going up one tooth or down one tooth hurts my laptimes. Either I max out too early on the straight and loose my pulling power, or gearing up looses a bit too much of the infield rip that I need. So, since I am WAY under the heat limit, heat doesn't even come into play in my gearing. Gear UP until my motor is coming off at 160 (Gearing for heat), would certainly decrease performance on the race clock.
There is newer guy at my track who runs 17.5 SC, and was worried that his motor was coming off too cool.. so he geared up 2 extra teeth... and his lap times were no better at all... but he was coming off hotter. So where he gained on the straight, he was then losing in the infield on the tight sections of the track. It is a net loss.
Now, that doesn't mean that for YOU, and YOUR gear, and YOUR track... wont get the best lap times geared tall and up against the heat limit. That very well may be the case.
So gear for performance/lap times... as long as it is under the heat limit. If the best performance keeps you coming off the track at 180 degrees, you need to gear down to save on temp, and find another way to improve your lap times. (smoother throttle, smoother brake, better cornerspeed, etc).
Just my $.02! Hope that makes more sense.