Last week DL3OCK generated a testfile for me. This file contains an FM signal and an D-Star signal with 50kHz offset. It can be transmitted with special equipment. I used this to check the behavior of my C5 70cm transceiver close to noise level and with impulsive noise. Find the videos below.
This video shows the comparison between D-Star and FM at around -115dBm.
It is clearly audible that D-Star has some advantages against FM if the decoder can stay in sync. The microphone of the camera does not represent the FM very well. In reality the FM sound was slightly better.
This video shows the performance of FM vs D-Star under impulsive noise conditions. It is clearly audible that D-Star works without problems when FM is hard to understand.