I've used Silent Coat on a D2. It did help on that because the original bitumen sheet was dry and falling off so probably wasn't that effective any more. Certainly there was a substantial difference in sound from tapping on the outside of the door skin before and after.
I still need to find a good solution for the inner door frame on the D2 as the plastic bag containing soggy carpet underlay doesn't really cut it
I will probably be using some Silent Coat on the door card itself at some point when I get the 6.5" speakers in there.