So I've heard betaflight can now overclock F4 boards. 
Does anyone know why do we overlock F4 when it's already idle at 5% cpu usage with 8k/8k and an accelerometer?

First of all, if you want to do 32K/32K you might need to overclock.
Secondly, adding dynamic filtering and 16K+ looptime you'll be over 50% easy! Then add blackbox, telemety, osd, ect. The new stuff is getting cpu heavy!

The downside is that the processor can get a bit warmer (even hot). The CPU speed is often limited by its ability to stay cool. Most of these chips are designed with passive cooling in mind while being crammed in a small device. Sitting in the open with good air flow should mean running them faster is really no trouble at all. At least to a certain point. Makes total sense to let us over clock them.

https://oscarliang.com/overclock-fc-betaflight/

If the FC has a gyro that can do 32kHz then overclocking could be worth it, but I think there is no point with a MPU6000 gyro that only has an 8kHz rate using SPI.


"The popular MPU6000 Gyro with SPI BUS (protocol) can have a sampling rate of up to 8KHz, while the ICM-20602 can do 32KHz." Oscarliang.com - Looptime and Flight Controller.