Hey Ken,
I ride a black bagger myself, a 2016 Road Glide.
Looks like we're in the same boat, trying to get slug velocities out of our dreamline .25's.
I had two goals, shoot 34gr jsb @ 880, and shoot 28gr or larger at more than 910 fps.
I found some of the same things that you have, reg pressure likes 110-120 bar, and anything more the velocity drops, even with hammer spring maxed.
The things I've done so far are returned internal hammer spring to neutral position, added Huma hammer spring, which got me to 850 fps and the gun shooting nicely, but only shooting 28 gr slugs at 875-880 fps.
I just added a second power plenum, that brought the 34gr JSB up to 880 fps.
I also found and sent for quote at fx usa the heavier hammer, I think this will be the key. I will return here when I get this installed. apparently there are 3 weights, 7, 11, and 13 grams, I weighed mine and it is 11 grams. The 30 cal comes with the 13 gram hammer weight.
I've read about valve seat diameter sizes, this is the difference between the dreamline and the other guns, and our limiting factor. Dreamline valve seat is 5.5mm, the crown and impact are 6.3, that is a huge difference of air flow capability.
I am going to continue this journey, I think I will end up with a maverick to shoot slugs, and tune the dreamline for 34gr jsb, which shoot crazy good at 100 yards, but get blown in the wind more than slugs.
Regards
Chuck