The S500 and S1000 are 15 meters in diameter, leading to 225 m² of coverage. The 4.8 m diameter wind turbine sweeps 18 m². With a wind speed of 10 m/s and a power coefficient of (Cp) of 0.37, the power would be 4 kW. You divide 225 by 18, which gives you 12.5 that you multiply by 4 kW: there you have your 50 kW!
But things do not happen like that. If it is true that the flow is constant in a Venturi, which leads to an acceleration in the narrowest part, it is also true that the Venturi blocks a large part of the flow.
Yesterday I conducted an experiment with a funnel (incomplete Venturi) of 25 cm in diameter, 14 cm high, and 3 cm in diameter at the outlet, which I placed against a fan: the airflow speed was 4.2 m/s while it was 0 around. Without funnel the airflow speed was 2.8 m/s but on the whole area covered by the 25 cm diameter funnel. In terms of power, we would have 2.8 m/s x 490 cm² (without the funnel but counting its base of 25 cm in diameter) versus 4.2 m/s x 7 cm² (the funnel and its opening of 3 cm in diameter). You are at a disadvantage.
The designers of wind turbine shrouds try to limit the ratio between the widest and the narrowest parts to get an advantage.