8000 / 60 = 133 1/3 per second, invert that to get period = 0.0075s = 7500 micros for the shortest period.
Is micros() being steps of 4 close enough for this use to just change the timing interval to get RPM?
Yo, you could give detail to the pulse, that's how slow to AVR the pulses are.
Write a bit to the PINx register to toggle the pin in 1 cycle.... there are 120000 cycles in 7500us.