Since I woke too early and have some time, I’ll give you my advice, albeit not specific to the P2, Matthias. I do not think that there’s a valve spring adjustment here but if there is, start out fully open/least tension.
Start with the reg at a level that is likely to be lower than needed (say 90bar) and follow manufacturers guidelines with regard to degassing (if needed) when adjusting the reg. Typically degassing is needed to reduce pressure (at the outset). Increasing pressure in subsequent steps can be done without degassing, but you should check!
You are obviously aware that tuning on the downslope for high efficiency can have some drawbacks. The extent of those effects vary from one gun to another. In keeping with your goal of quiet operation, it’s worth noting that once you get to 5% below plateau speed, the gun should already be pretty quiet. It may not be advantageous in terms of noise to drop further down the curve, so I would first aim for a target speed that is 5% below plateau. You will get a sense of sound in relation to the curve as you tune and can adjust the target as you see fit. One issue with tuning below the knee is that projectile speed will tend to follow tank pressure. Regulators are imperfect and tend to run higher at full tank and vice versa. The optimal tune (at -2 to -4% below plateau) buffers that effect and therefore losing that buffering is a side effect of what you’re planning to do. Therefore, I recommend doing your tuning at median tank pressure. For example, if your max fill is 250 and reg ends up at 125, do your tuning at 190bar, either tethered or by keeping it in that range (210-170). Otherwise you can end up with a gun that shoots too fast or slow at one end of the spectrum because you tuned at the other end. Onto the process.
At starting (low) reg pressure and low hammer, run 3 shots over the chrony and incrementally increase hammer and repeat. Velocity will increase with hammer until you get to the plateau speed for that particular pressure. As you approach it the gun will get louder due to wasting air.
Determine if that plateau speed -5% is what you’re looking for. If not, increase/decrease the reg pressure by 5-10bar (depending how far off) and repeat the process. Since increased hammer is needed for increased pressure, you don’t have to go back to minimum hammer to find the plateau after increasing the reg - just back it off a bit to find the new plateau.
The goal is to find the pressure and hammer setting at which you get your target speed at plateau speed -5%. You should now have a sense if that 5% drop is sufficiently quiet for your needs. As you dial it in, take more shots to determine how stable the speeds are and try to do that at the median tank pressure.
Once you’re set at plateau-5%=target speed, fill the tank to your intended max working pressure and test the speed to see how much it perturbs. Then do the same at a pressure of 20-30bar above the reg set point. Is the deviation acceptable? If not then you have to go closer to the plateau speed.
Finally, if there is a valve spring adjustment to fine tune dwell, you can increase its tension progressively while checking speed until it just begins to reduce speed. In addition to make things a little quieter, it can buffer the effects of variance from the changing tank pressure as you shoot.
Hope that helps.
PLEASE, 
You guys are really great! 
