Rough running brand new 912uls

The Enricher Fuel Circuit (Choke!) is only effective in the lower RPM range.
It has no effect either way above 3000 rpm.
Check the chokes.
Either One or Both of them may be engaged all the time, giving you a bad time below 3000 RPM.
You may have had a cable slip.

Likely not immediately helpful, but i wish I had the kind of spare change that allows one to rationalize taking a brand new ULS up to 5.7 K,  ... knowing that its not running right !   

I agree with Bill – check the choke circuit.

Also, try balancing with actual vacuum gauges instead of a "Carbmate".

I plan to check the carb balance tomorrow but I don't have strong hopes that is the problem.  That's because my throttle is at the center console (Searey) and has a separate cable to each carb.  I can stretch each one from the cockpit to increase the throttle on each carb.  Doing that does not smooth out the rough running a bit.  I'll also check that the chokes are closing while someone else operates the choke lever.  However, looking at the engine while it's running shows both enricher levers fully against the stops.  

Food for thought and trying not to leave any stone unturned. 

Just had an RV12 with a similar issue. It was a choke 180 degrees out of position on the left carb. Just to eliminate all possibilities  take a look at the end of the brass choke shaft. There is a center punch mark on each one. It should be facing towards the air filter intake side. Another way to look is the aluminum choke body that's held in place by four screws  and looks like a molded aluminum pipe is coming out of the air intake side. That is the way the center punch mark should face. If by chance it was reversed on either carb take off the choke lever and rotate the brass shaft 180 degrees to face towards the air filter intake.

You can also clean the idle jet if you want to take a look. When you drop the bowl you will see the main jet hanging down. Right next to that there is a hole and up inside is the idle jet. Use a straight tip screwdriver to unscrew it.If you hold it up to the light it should have a nice small hole through the center. Take a 6" piece of wire and strip off about 4" of the insulation. Then unwrap a single strand of wire. You can then slide this down through the center and slide and rotate the idle jet on it cleaning out any dried fuel or debris.  This won't hurt it. Been doing it this way for 20 years. Then flush it out with some spray carb cleaner and then follow that with some high pressure air. Then replace the idle jet and carb bowls.

Then do your pneumatic carb sync. Use gauges instead of an electronic device. You can diagnose, troubleshoot and sync carbs easier with gauges.