I got you David, except even a hole has to have a cover and I doubt the wide slot openings in the turbine as well as the gap between the stack and the spinning turbine would obstruct the opening much more (if any) than the flat cover of a standard flat vent.

Then add the screen mesh in the standard flat vent which is not present on standard turbines and I think the energy argument is pretty much a wash.
Then add the fact that the exiting air spins the turbine AFTER it has exited the stack and is past the lower gap, and you see it is the energy of the rising air in the atmosphere, not the pressure of air pushing out of the attic that spins the turbine, and therefore no energy lost to the spinning of the turbine.

__________________
Jim Luttrall
Mr. Inspector.net, Inc.
Allen, Texas 75002