<<Once your waste gates are fully open, that is above about 21,000 ft
you may want to change some engine settings.>>

I think you meant "fully closed."  Yes the fuel control, even though it is
not altitude compensated, thinks it is at a constant altitude if the
manifold pressure is constant.  There will be a slight leaning effect with
climb as the back pressure drops, making the volumetric efficiency of the
engine increase slightly with increasing altitude.  Certainly at
higher-than-cruise manifold pressures you don't want to tinker with the
mixture, but at cruise manifold pressure I don't see any reason not to lean
to a cruise condition.  The engine doesn't know whether you are climbing or
cruising - except for cooling and I don't think cooling would be worse in a
low-altitude climb that it would be at a 25,000 foot cruise condition.
Again, I don't have any IV-P experience.

Gary Casey