They can help in certain situations and setups. I had one for my SJ, but with a bored 85mm pump nozzle I was loosing some needed pump pressure. So i put the stock cone back on and it was better. On SJ's I say do one or the other but not both.
I dont have experience with the cones on Kawis, but if I were you I'd try one. I'm sure it will help your 750 out.