I've done the Death Ride 5 times and I've heard the 7 and a half hour hour Gary Fisher rumour before, but I can't say one way or the other whether it's true or not. I've done a 7:43 before (2004) for all 5 passes - that got me second finisher of the day to sign the poster, but it's difficult to tell because it's not a mass start event and riders can leave whenever they choose. Most of the fast riders I know start at 5:30 when you are no longer required to carry lights...
The fastest time I know of that I can verify was 7:35, but since riders' times aren't recorded by the organizers there may well be riders who have done it faster. However if this is the case then they've have kept it pretty quiet.
I'd say from my personal experience that a sub 8 hour Death Ride is a pretty good achievement.
I didn't get in this year :-(