OK, I've fiddled with this enough today. I can make it do 750s with 16 grain pellets, but for 10 shots and horrible accuracy past 15 yards. Or I can dial it back into the 575 fps range and have good accuracy, but point of impact keeps rising because no matter how I adjust the spring or pressure, it's just too much curve.
I'll have to hit the hardware store for some different springs to try. The stock spring is long and very stiff. I need something either of lighter gauge, or even stiffer and shorter to make the hammer free flight.
Testing at 50 cents a mag is getting costly, but of course this thing has expensive taste. Why can't it just like crosman pellets?! Lol.
Other options are finding a regulator that fits it, or as I mentioned before, restricting the transfer port on the valve. That has worked wonders for me on the crosman 22xx and QB78 platforms when I had wonky shot strings.