It will be interesting to see what the real experts say, but it seems to me like the waste gate is blocked from completely closing.  Or there is physical damage to the turbo itself (broken blades or something).  I'll put my money on the waste gate.

Gary Casey

On a recent flight to Chicago, my MAP during WOT climb was normal and ample all the way to cruise FL220, at which time I pulled throttle back to 31.5", dialed down RPM to 2350 (which increases MAP some, so readjust) and "Big Mixture Pull" to LOP 16.4gph. All very normal. Cabin Pressure 5 psid.

On my return, departing during 30*F temps and cleaning some overnight rain-ice off the wings (via hanger heat), I climbed out of Chicago WOT and had full MAP at departure, but then observed a decreasing MAP during climb (numbers are below). Details below.

I leveled off at 17,500 and observed WOT ~31" MAP and RPM 2690, 32.3 gph, all engine parameters normal. I did not reduce throttle, I dialed back RPM to 2350, and observed MAP reduce to 24" (and cabin altitude climbed with 1.5 psid).