Any reason the RV-12 POH performance section only includes performance data up to 8000 feet pressure? I would think up to 12,000 at least since people do use these through the mountains and it is rated for it. I don't mind extrapolating the numbers but would be nice to see it on the charts.
I've heard that the Bing Constant Velocity Carburetors, usually used on BMW motorcycles, do an adequate job for compensating for an Altitude of up to 8000 ft and beyond that, they sort of run out of adjustment. Add to that summer time heat, and DA's of +2000 to +3500 ft above what you're flying at, and the situation becomes more understandable. Carburetor runs out of altitude adjustment.
YMMV, this is just what I've heard from quite a few sources, both Rotax airplane and large BMW carbed motorcycle engines and their owners.
You do want to run the throttle so that it's
NOT full rich, needle valve completely out of the main jet seat, throttle fully up against the wall, while in cruise mode. Just runs rich and blackens your plugs. Backing off of the throttle 3/4 to 1" away from the wall allows the mixture adjustment to work, full rich position is generally for climb out only.
Edit: I see your engine is Fuel Injected FADEC. I don't know the reason why. I thought they ran out of climb rate at around 14,000 ft.