Once at a boat show I asked a Beneteau sales rep why fuel consumption of the ST50 is higher than that of ST52 according to the Boat Tests, which Beneteau was quoting in their marketing materials. The sales rep appeared to be surprised, started checking with his iPad, but could not find the answer. Took my business card and promised to get back to me. That was 3 years ago and I never heard from him since.
Major efficiency factors affecting fuel economy are, in descending order of importance, propeller efficiency, hull efficiency, and engine/transmission efficiency. Since hull and engine efficiencies should be relatively close between ST50 and ST52, the IPS propellers of the 50 must be responsible for high fuel consumption. If the Boat Tests numbers are correct.