First : nope, nothing audible.
Second : yes it adds an adequate amount of sustain... it won't go into comp-sustain overkill though.
Lastly : demos sound great, never tried one though
I love the Diamond, and it's been on my pedal board for 3 years solid. But being the comp junkie I am, sprung for a MP Forest Green yesterday, so I can give you a comparison shortly. Also would like to try the RAF Mirage, Empress, and Strymon. At times I want a lot of squish for country, and other times I think the 'sustain' mode would be advantageous with the MP.