I guess I'd start with "it depends" on the droop setting. Usually 3% is considered pretty standard, I have recently setup some larger units at 2% per the customer request, a newer design digital governor allowed very good tracking after getting it adjusted so for that application it appears to have worked quite well.
Larger droops are usually applied to industrial drives and engines driving equipment, typical on-hiway truck engines with hydra-mechanical governors had droop from 6 to 10%, about the same with marine propulsion engines.
I rarely apply speed droop settings in systems I work on as they are all isochronous load share systems using a Woodward, Basler, ComAp or similar load control systems, especially on engines with integrated governing in their ECM's. In some cases if required and the controls are setup for it we apply 3% speed droop for a manual control mode if the engine controls can use an input to switch in and out of droop.
On the steady state frequency, the spec usually comes from ISO 8528-5, performance class (grade) 3 is at 0.5% for steady state frequency. Depending on engine size and things like fuel systems and turbo's, steady state frequency can be improved but usually at the expense of poorer transient response, at least in my experience.
Hope that helps, Mike L.
