I thought an airleak caused things to lean out and get too much rpm.
The saw sounds snappy and great. I hear the 4 stroke cleaning up. I'm suprised that the tach stays the same as well.
The fact that it hits 14k when it's ice cold leads me to believe you aren't moving enough air/charge when it's warm.
What's an 056MagII run stock in terms of rpm? Max rpm is generally based on jug design, give or take.
Most hybrids run higher rpm, not lower, like the common 046/044. You have less case volume, not sure which case you used.
I'd change the spark plug, run without an airfilter and see what happens. If there's no change, take a look at the jug, the timing, and how the piston and lowers breath at BDC. I know you know your stuff and I'm sure you've studied this already. If all else fails, it's gotta be jug design or some drag on the motor at high rpm-like a bearing that's binding or a rubbing flywheel.