I'm not sure if this is a bug or a game feature, and I don't fly the B-26 very much, but the flight power seems to be porked. The Manifold pressures drop off rapidly with altitude. At 13K it will hardly climb at all, barely 250FPM.
Here's the max manifold pressures (no wep) I just recorded:
At takeoff from Sea Level 54"
Take Off from a 5k base: 42"
6K: 40"
7K: 37"
8K: 35"
9K: 33-35"
By 13k you're at 27"! If you hit WEP it will pop to 44".
According to E6B, Normal is 42" and WEP 52".
The result is the B-26 has extremely poor performance above 5K.