A way to get a higher Cp than the Betz coefficient is to surround the rotor by a venturi. However, an effective venturi must have large dimensions in relation to the rotor diameter. Especially the part behind the rotor must be very big to make that sufficient under pressure is created to suck a higher mass flow through the rotor. So an effective venturi is very expensive and one can better put that extra money into a bigger normal rotor. Some manufactures supply windturbines with a very small venturi and this might give a little increase of the Cp because the tip losses are reduced but one should not believe the claim that this venturi gives a strong increase of the Cpmax.
Long ago there was a project gouverened by Theo van Holten of the University of Delft which claimed to increase the mass flow by using special tip vanes at the blade tips. Wind tunnel research has demonstrated that indeed the mass flow was increased. However, the increase of the power due to the increased mass flow was completely consumed by the extra drag of the tip vanes and finally the Cp wasn't larger than for a normal rotor. So this project was cancelled.
The maximum Cp of a real rotor is much lower than 0.59 because of four reasons: wake rotation, tip losses, airfoil drag and an effective blade length k which is shorter than R. These effects are explained in chapter 4.3 of my public report KD 35. Airfoil drag is the most important effect for fast running rotors used for the generation of electricity as these rotors have a rather high optimum tip speed ratio.
Figure 4.7 of KD 35 gives the theoretical Cpth value as a function of the optimum tip speed ratio and the Cd/Cl ratio for a 3-bladed rotor. In this figure it can be seen that Cpth = 0.445 for a rotor with an optimum tip speed ratio of 6 and for an airfoil with a Cd/Çl ratio of 0.03. However, this Cd/Cl ratio can't be realised for the whole blade length R but only for a length k. Formula 6.3 gives the reduction of Cpth into Cpmax because of this effect. Depending on the ratio k/R, the final Cpmax will be in between 0.4 and 0.42 for a 3-bladed rotor with Cd/Cl = 0.03. In the same figure it can be seen that Cpth = 0.48 for Cd/Cl = 0.02 and 0.51 for Cd/Cl = 0.01. These very low Cd/Cl values can only be realised for big wind turbines which have large chords and run therefore at high Reynolds values. Very low Cd/Cl values are only possible for high Reynold values. So a real Cpmax of 0.5 is the absolute practical maximum.