Tuning Air gun Regulator

Short answer yes when you drop to 150 bar you should fill up again 

If you type in "how does an airgun regulator work" in Youtube you'll see several helpful videos to learn more.

For the most part;

Yes, higher reg pressures can mean more power. But that can depend on your barrel length, valve and barrel porting and hammer spring settings.

Yes, lower pressures can mean less power but depending on the above.

Example: I increased my reg set point from 130 to 140 bar. Will the gun now shoot faster? Typically NO, it will shoot slower. HUH? Well, yes because there now is more air pressure shutting the valve faster so that less air is released to propel the pellet ... UNTIL i also increase the hammer spring tension so that the hammer hits the valve stem harder, so that more air can be released behind the pellet than before. Only then does my power increase with the higher reg set point.

Likewise, a reduced reg set point often increases the pellet speed (and air wastage) to an extent until the hammer speed tension is reduced as well. In other words, reg set point and HST need to be calibrated in function of each other so that the gun is tuned "below the knee". I recommend the excellent PCP tuning blogs by Bob Sterne.

With a traditional knock-open valve, yes.

With a "balanced" valve, the higher the pressure, the faster the velocity.

At least that's the way the Cochran Powerhouse valve works.