" Conclusions
1 Despite only a narrow class ~1-parameter family! of the flows
has been considered for optimization, the result obtained allows us
to conjecture that the efficiency is maximal when the resistance is
rather small and a large part of the flow ~61 percent! goes through.
In other words, the maximum efficiency could not be noticeably
greater than what was obtained here.
2 The model of a free-flow turbine reveals a new class of problems
about streamlining with partial penetrating through an obstacle;
some of these problems could admit explicit solutions and
could have other applications.
3 The velocity of a flow vanishes at the origin of the proposed
plane model. This makes the model specifically applicable for
two-dimensional propeller-type turbines in free ~nonducted! currents.
The theoretical limit of the efficiency given by the model is
30.1 percent. A number of tests, as well as constructed power
farms, support this thesis in regard to both hydraulic and wind
applications. The efficiency of most water and wind propellers in
free flows usually ranges from 10 to 20 percent. On the other
hand, the three-dimensional hydraulic helical turbine develops an
efficiency of about 35 percent in similar free flow conditions @2#.
This high efficiency might be explained by modeling a 3-D rotor
as a combination of two plane turbines that reflect power contributions
from the front and back parts of the original cross-flow
turbine."

What I find significant is the "35%" efficiency claims for the Gorlov design.

Also here is a commercial wind power helical reference: http://www.turby.nl/downloads/Turby%20ENG.pdf