You have to look at it like this..
When you install a regulator that's set 100-130bar or so, you are basically shooting the gun as if you were right at the end of a fill before you installed the regulator. And that's where the gun is the least efficient, at the end of a fill. You have to tune the gun around this, and sometimes accept less power.
The quick way, is to set the reg to be near the sweet spot of a fill when it was unregulated. Shot count won't be great, but the power should stay the same with decent efficiency since this is where the gun was optimized in stock form. If it's not, you likely don't have enough volume in the plenum that holds the regulated air charge. It's better to have the plenum too big than too small.
When tuning for lower pressures, you will benefit from a bigger transfer port as well.
The other big killer for efficiency with a regulator is hammer bounce which usually goes up at lower pressures. I believe your gun has the Harper Slingshot system which would make this much less of an issue. So that's good news if that's the case.
When it's tuned efficiently, you won't see the velocity drop drastically when it comes off the reg, and there won't be a huge velocity spike either. It should taper off gradually.
I'm not an expert, but those are some things I've picked up. The biggest thing I learned is that a regulator is NOT a drop in affair, despite whatever the seller may tell you.
Again, just spitballing here.