I had read somewhere that it's a good idea to set a throttle limit of 90% for racing. The reason given was that, in the last 10% of the throttle, the cost/benefit ratio for power to thrust would not be worth it. Is there any truth to this? If so, what is the recommended throttle limit for any build/all builds?
The reason why this is recommended for racers is likely due to the fact that during these high-throttle punches, the voltage of the battery sags from the load and so more current is pulled from the battery to increase the motor RPM.
Power is equal to
V is voltage and
I is current, which can be rewritten (by substitution of Ohm's law
V = IR in for
I^2 * R. This means that if more current is drawn to compensate for a voltage sag, the power consumed by the motors increases by a greater amount because the resistance of the circuit is constant. (for all intents and purposes) For example, in the hypothetical scenareo that twice the current is drawn, four times the power would be consumed. This results in quicker battery drain.
The 10% figure that you've heard isn't a hard line, but rather just a suggestion based on experience with miniquad hardware and its capabilities. This recommendation isn't just specific to racing; all quads would benefit from the efficency (energy extraction from the battery) boost from rarely using the top XX% of the throttle range. However, this is often more important in racing scenareos where it is often necessary to make the most of the energy in the battery pack you're using because you've selected the smallest and lightest one possible and need as much energy as you can get to complete the course in record time.
There are two reasons to throttle limit, one is obvious, the other is not necessarily obvious.
The first reason is that both thrust and amperage draw grow rapidly at higher throttle values, and particularly because of the interaction of battery sag and the wattage motors will pull at the top end, it is possible to exceed the amperage of battery and electronics operating efficiently (or operating without letting out the magic smoke). For this reason, the general rule of thumb for racing 5" quadcopters is that the point beyond which 38k RPM would be met is excessive, and unlikely to contribute meaningful thrust beyond that point. Since carrying a large enough battery to provide extra power is usually a significant weight offset (greater than 5% craft weight to jump up to the next highest capacity within the same battery voltage and discharge lineup.
The second reason is less obvious, but very relevant for racers - resolution on the sticks. By properly limiting throttle output, it is possible to create better resolution on the control gimbal by having the full stick travel focus on a narrower section of the available throttle power. Correctly is important here, you don't want to limit throttle in a way that converts the top N% of stick travel into a dead zone. For me personally, I run a 94-96% limit at the transmitter, even when I want full throttle, because at these input throttle levels, at least one motor will be at 100% because of the PID loop compensating for some attitude adjustment while flying, and the way that mixer works, I can't actually get more overall thrust out of that quadcopter because it has one motor already maxed out.
Finally, running 'excess' KV setups where, for example, you are leveraging the ability to run 2500KV batteries on 5S or 6S packs that you can take advantage of the higher voltage and efficiency of lower amperage draw in most flight regimes for running those batteries, you can limit the motor output (again, 38k RPM is a solid guideline here) you will want to limit the motor output at the flight controller (i.e. never ask the ESC for a throttle setting above a value at which the ESC or motor will hurt itself, desync, overdraw batteries, or turn the propeller into a audible goose-honk source), then you can work to add resolution to the sticks through that same method if you so choose.
Resolution is the less obvious, but often more valuable reason to focus on this for racing applications, because having more stick travel range to make fine adjustments in will allow for more accurate corrections.