Issues THC.

Try P=5 and post picture.
Also, watch the voltage on screen, does it fluctuate a lot? +-10V is OK, even a bit more is still OK, but 50V difference is not OK and might be interference or loose contacts on the plasma side.

Yes it is, but that also depends on a lot of other external conditions, like current set, air supply and pressure and moisture in it, nozzle size and type, feed rate, material composition, etc etc, so any of change and the cutting result will change, most of the time not noticeably but sometimes very noticeably.
If you have an older plasma source with a big transformer, that will also vary with the input voltage and spikes and drops in the line ripple.
As a general rule:
-thinner material or faster feed rate = more P
-thicker material or slower feed rate = less P
-
On most of my machines i have it set at 7 and never change as they do mostly cut the same 2-3mm material all day long but sometimes they do cut 6-10mm thick so it works fine for both materials as the wavy cut is not very noticeable on thin material.