ATMEGA328P Max Clock Speed Without Divider
If you open up the chip datasheet, the maximum frequency of Atmega328P is 20 MHz. It will need at least 4.5V tor run at that speed. And if you want it to run at 20MHz with a 20MHz clock, the clock divider needs to be 1 so it does not run on slower clock than 20MHz.
You can't change the clock speed by changing F_CPU. The F_CPU tells to the compiler and libraries what speed the CPU is actually running, so you must first modify the hardware to run at the clock you want and tell what the clock is to the software by setting the F_CPU accordingly.
Also, overclocking means running the chip outside its intended parameters. If you overclock it, there is no guarantee if it works, and if it does not work, what part will fail first.