My experience with the IO550 differs somewhat from yours. Since the IO550 has the same specific fuel consumption as the IO520, if you wish to duplicate the performance that you had with the IO520, all you need do is adjust the power so that you have the same fuel flow at the same HP. I typically operate LOP if I am at altitudes below about 13,000, so it is easy to dial the mixture back to achieve whatever fuel flow I desire. However, I find that I can usually make a trip using less fuel that I did with the IO520 by climbing 2 to 4 thousand feet higher and obtaining a higher TAS at the same fuel flow.
Bob Siegfried, II