ipmitool tells me that the PWM fan I put in the rear of my case is running at 1800 RPM
Noctua rates it for 1500 RPM +/- 10%, that suggests that 1650 RPM is the maximum
PWM, by definition, can't overclock or overvoltage anything, it simply increases the pulse width up to the point where pulses are 100%. You can't PWM over 100%
Is the fan simply running outside the spec or could this be a reporting error?