More factors then just the manifold govern the turbine 'behaviour' such as turbo design, PSI, engine design, displacement, cams, Volumetric efficiency and off course witchcraft.
But your theory is no backpressure is the best, so have u tested this on a dyno by running back to back with an exhaust and then without dump pipes to run straight of the turbo like u said?
I'd be interested in the results..