The above graph shows the power coefficient curves for the best uniform pitch angles and a blade twisted as in the baseline turbine. Twisting the blade appears to have the effect of reducing CP below the velocity for which it is optimised, and increasing CP in the region of that velocity and above. In this case a 6.7% increase in power output was seen at 2.62m/s. Over the course of a year this resulted in a 4.8% increase in energy recovered from our hypothetical tidal cycle model. For a farm of 24 turbines this equates to 29.54 MWhrs per year. Twisted blades are likely to be more expensive to manufacture so you must ensure that the return on your investment will be worthwhile.
