Neither automaker underrated or overrated the outputs of these cars. All of the published C7 Z06 test results were for the Z07 version, which makes it more comparable to an AMG GT R on the track. AMG has been known to build track cars for straight line use.
AMG GT S is more like Ferrari F430 or the Corvette Z06 that came before this one, but with even quicker shifting and a much better torque curve.
The AMG GT liftback sedans are underrated for sure, but the two-door cars are usually within their horsepower figures.