if the wastegate is open the mean exhaust pressure is maybe 1.05 bar
if the wastegate is closed the mean exhaust pressure is maybe 4.05 bar
even with an active turbine surely the 4.05 bar stream has bigger pulse reflections available than the 1.05 bar stream does ?
what did the 80s F1 turbo engines do ? (though they might have had no use for the benefit as they didn't recover power)
ok the 4.05 bar stream the pulse energy has less energy relative to the steady flow energy (than does 1.05 bar stream)
because less of the 4.05 stream's flow will be choked as the gas acceleration will be less due to the higher density