I'm not aware of a Shimano 9sp rear mech with a clutch - I think that came in with 10 speed - but my knowledge of Shimano derailleurs is far from exhaustive so I may be wrong. The only advantage I can think of for using a 9sp MTB mech is that they have a longer cage so you can run a bigger cassette. That's why I used one - I wanted to be able to run an 11-34 cassette with a 30-39-53 triple at the front, and needed the long cage to do so.
Not sure I'd call it racing, cos it's not, but I used that setup for the Fred Whitton for three years. Shifting was fine, and no problems with chain slap.
I'm not aware of a Shimano 9sp rear mech with a clutch - I think that came in with 10 speed - but my knowledge of Shimano derailleurs is far from exhaustive so I may be wrong. The only advantage I can think of for using a 9sp MTB mech is that they have a longer cage so you can run a bigger cassette. That's why I used one - I wanted to be able to run an 11-34 cassette with a 30-39-53 triple at the front, and needed the long cage to do so.