I find it hard to believe that the use of headphones (in an excessive volume state) IMPROVES any pilot's awareness. Like any rule, there would be exceptions. Perhaps one could achieve more visual focus. However, how would one's auditory awareness improve?
It is reasonable to assume that the majority of pilots would have diminished overall AWARENESS.
I would also wager that some pilots' awareness would be more greatly impacted than others'.
It is very reasonable to highly suspect that the jackass who rammed the bike was not paying reasonable attention, and was further handicapped BECAUSE of the headphones.
I am not against the use of headphones if the INDIVIDUAL is CAPABLE of maintaining situational awareness of AT LEAST an EQUAL state of when NOT WEARING headphones.