My suggestion would be to sharpen the blades and keep the deck clean. Making the blades rotate faster than they were engineered for is a safety issue and a probable quality of cut issue also. By creating excessive air flow that your deck is not designed for will most likely lay the grass down before the blades come in contact with it, ultimately leaving a ragged cut.
Properly sharpened and balanced blades. Engine ran at Wide Open Throttle. Only go as fast as the mower can go and still manage a clean cut.
If you want a faster mower, then you need to invest into a premium commercial mower.