There is a theory that the schulz nozzle does allow the prop to be more efficient I dont know exactly how, this effect is used by this type of prop guard to make it in theory at least, able to operate without loosing speed. It is supposed to be a tunnel wing used underwater I think.
I would hazard a guess though that the increased appendage drag does means speed will be lost at higher speeds. Though slow speed manouvrebility may be better.
Again it really falls to more exhaustive testing to prove either way or an expert in the field to shed more light, my green blobless status is sadly a reflection of my paper thin veneer of knowledge.
New boat is here, very happy!