Hey Christian,
I read through your swap post today, great looking XR you have there.
1. The sag on the forks is about the same. ≈1". Also, my bike no longer does the big nose dive on braking like it did before.
2. Yes, my bike just feels more stable with the USD's It handles the bumps and ruts better and is just more planted than it was. I am definitely able to go faster on the same sections of trail than before. I am not riding anything at crazy race speeds, I ride for fun. I ride a mixture of street, dirt/gravel road, and single track trails.
3. I cut one coil out of the fork springs and added a spacer to take up the space left by the shorter spring. Shortening a coil spring stiffens it, changing the spring rate, not just increasing the preload like just adding a spacer would have. My springs are approximately about a .46 kg/mm or so. I resprung the shock as well with a 12.0 kg/mm Eibach spring. I rebuilt the forks and shock but no valving changes, just new seals and fluids. For the riding I do I have enough adjustment in my clickers front and rear that I don't think I'll need to revalve anything. I'm loving my new setup. My race sag sets up perfectly front and rear for me with these springs and the bike is performing well for me.
The headlight is awesome, very bright but not offensive to others on the road with the high-low feature.