I'd be interested in seeing your disperser. I'm pretty sure the noise is pretty much exhaust driven as the cannister noise was a lot more tolerable and the level was very acceptable. One guy at a contest thought it was a ys, the land owner of our local club said he thought it was as quiet as the electrics. I'm also thinking a prop with a little more load could help change the exhaust sound. Hard to find a carbon prop to fit the bill. I'd kinda like to stay away from the apc as I don't need the nose weight. PT makes a 21x10 I may give a try.
I've avoided the soft mount because of the added weight. Another 3oz up front coupled with the 4oz weight of the motor extensions would be tough to overcome. I've already moved all servos to the tail. I also feel that's why we see so many broken headers. I've been trying to think of a way to get rid of the motor extensions. That would help a lot.