Whereas the original poster said nothing about a BMEP (Brake Manifold Effective Pressure) gauge, it need not be addressed here. It will show a power loss, but as it hasn't been included in the instrumentation stipulated in the original post, it's irrelevant.
Pedant mode on-
BMEP = Brake
MEAN Effective Pressure - Is a theoretical pressure developed inside the combustion chamber that if maintained equally throughout the two or four stroke cycle will develop a set horsepower. It has ABSOLUTELY NOTHING to do with MANIFOLD PRESSURE!!
Someone suggested that because the supercharger is geared to the engine, and because the supercharger is theoretically turning at the same speed as the engine, there will be no manifold pressure loss. This isn't true. Bear in mind that because of the use of a constant speed propeller, one can increase manifold pressure substantially above barometric, with the use of a supercharger, with no increase in RPM. RPM, therefore, is not the only determining factor regarding manifold pressure, when a supercharger is used.
Nope, RPM is not the only factor governing manifold pressure but it is the only factor that governs supercharger output and with a constant throttle position, which is supposedly what we're dealing with here, would be the only thing that would affect manifold pressure!
And boosting manifold pressure above barometric is the whole point of the thing!!!!!!!
Pedant mode off!