Get thee a look at ANY radial engine power chart to see that your assumption is not correct. Almost all radials have blowers, and spec a lower MP for a certain percentage of power or HP output as altitude increases. It is widely known that the reduced exh back pressure at higher altitudes is the reason for this.
Example, using the good ol' 985: setting 220HP output as a data point, at 5000'PA, ambient temp 10C, 1700RPM, the engine needs 24.7"MP to produce this power. Move to 10000'PA, 0C, 1700RPM, the MP drops to 23". At 10000'PA/10C, it is 23.5". 24.7"MP @10000'PA/0C/1700RPM = 240HP, a gain of 20HP.
BTW the 985 is power rated using a constant 32-38C induction temp, so in most cases, this requires carb heat. Seems this might take the ambient temp out of the equation if used as directed by the folks at Pratt.
As for that power robbing blower, the fuel specifics on the blown 540s seems to equal the normally aspirated ones -- determined by side-by-side flights of more or less equal airframes and comparing fuel flows during those flights. So, while the blower (either exh or mechanically driven) must take some power to operate, it makes the engine think it has a higher compression ratio, which is more efficient at converting refined dinosaurs into speed.
The nice thing about superchargers is that they have no TIT temp restriction, which causes turbos to be VERY thirsty in the ROP regime for a given power output.
To confirm or deny what I have said, I'm sure Tom will have a NA engined RV fly next to the supercharged one to run the comparison. I'll bet the results will be positive.
Carry on!
Mark