I'm with some smart engine people next week. I'll ask. But I don't like the back pressure argument. I think if it was reduced backpressure the effect would not drop off as quickly past 12,000 ft. Also - have you seen the amount of exhaust pipe after the turbo? In the Seneca it about a foot of straight, relatively large diameter pipe.
Also, assuming the manifold pressure gauge reads gauge pressure and not absolute, then its essentially telling you the pressure differential between the inlet manifold and atmosphere, ie its incorporating a measure of back pressure.
I think if the engine develops more power at 12,000ft with a 40 inch differential between inlet manifold & atmosphere than it does at ground level, then something else is at play.