Hmmmmm... add a regulator?
IMHO, the SD and ES numbers on an unregulated PCP don't mean much. True that there's a somewhat "flat spot" where the pressure and valve balance but you are really dealing with a constantly changing situation.
A variance in velocity will affect the trajectory. If you're shooting tin cans at moderate ranges, a large change in trajectory over the whole shot string is not a problem. If you're shooting for precision groups at longer ranges then changes in trajectory are critical and only a small portion of the curve is usable.
I'd suggest that you tune for best pellet performance. work within the part of the curve that meets your needs and don't worry about some arbitrary numbers.
SD are ES are good points of REFERENCE for the performance/consistency of the valve/power plant but not necessarily an absolute indicator of accuracy.
The regulator in a PCP saves me the trouble of plotting the power curve and constantly monitor/adjusting the fill pressure. Call me lazy but I prefer the convenience of a regulator even if they are potentially a source of trouble. (I prefer automatic transmissions in cars as well
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)
Just my 2 cents.
Cheers!