I added foam between the motor mount and the chassis, in the space created by the H-plate ... then I stuffed some more under the "hightail" rear end of the LeMans Porsche and it seemed to help -- you need to fiddle with the amount of foam to get just the right amount -- I wonder how it compares to this Atomic part?
I do agree that this is an area where the D'NaNo needs significant tweaking, rear dampening is something I struggled with all last night.