I get in trouble every time with this stuff, I'm not an engineer.
But yes,, if your motor is actually overheating, then as it starts to melt its winding resistance increases, and you do start to feel it in the power, just before the flames shoot out the holes you drilled in the cover to help it cool slightly faster. You feel it lose power as it gets flaming hot. If you drilled cooling holes in the cover, you smell it before the flames shoot out the holes. But in these cases, you were never running at 85% efficient. Melting the motor starts either with overloading the motor, or over powering it with more watts than it can handle for long.
My experience though, after melting quite a few motors, is what really helps your motor is to stop in time. I found even a tiny 1/8 inch hole drilled in the cover let just enough smell out, to let me know its time to stop and let er cool down.
The short answer is don't overcook your motor too much, and it runs cool enough to keep its resistance down. What matters is time to overheat. Not overall efficiency. I did a lot of real world testing of time to overheat, which I needed to be at infinite, to make it 60 miles or more in the rocky mountains. The key thing was hitting an equilibrium temperature, vs running so inefficient that you had a one hour, or half hour time limit. Example, 500 w rated motor running 4000w had a time limit shorter than a 30 min race. The motor flamed with a lap to go. Wouldn't have won with 3000w, but would have finished in the top 5. Bigger motor would have lasted long enough.
Much depends on your needs, but whether you want a one hour ride or an 8 hour ride, you need to make the copper in the motor big enough to last the time you need, even if less than 85% efficient. To get infinity, its more a matter of making the motor big enough to be at high enough rpm to stay in that max efficiency zone where the motor will cool as fast as it heats, and reach equilibrium temps that don't roast the winding. Its matching the motor size to the load, and or the power level you plan to push.
Typical 500w DD motors common in cheap kits handle 1500w with ease, unless you weigh way too much. My eventual solution to infinite time to overheat was the old clyte 5300 series motor running only 2000w. It was plenty to get me and my cargo over the mountains at 15 mph, remaining efficient. So the 100 pound bike, my 200 pounds, and 100 pounds of cargo never even got warm at the top of a mountain pass in the rockies. Even when the day was 100F! I had plenty of copper in that motor to push 400 pounds up a mountain, and remain in its maximum efficiency rpm. Hard to improve on max efficiency, and the motor covers would never even feel warm, let alone hot.
I overkilled the motor, but then ran only what I needed to get up that hill. Sure, I could have run 4000w, and got over the hill faster. But at 15 mph I was running max efficiency, which is what I needed to cross 50 miles of desert on the other side of that mountain.
