The test flights use an A8, but the design seems more suited for a B6 or C6. So you're looking at ~1 minute dead air time for that model on a B. Decent, not great.
The biggest two factors in any duration event are altitude and decent rate. Trading one for the other is a matter of style and personal preference.
I think that the times shown in the video could be vastly improved upon with a little tweaking. The oscillating shock cord steals a lot of energy from the system, which increases the decent rate. My guess is that the hub was seriously out of balance, which is what drove the oscillation. The unbalanced hub also increases friction on the shaft, stealing more energy still. You'll notice that the airframe was spinning as fast as the blades; the airframe should turn slowly if at all. If you're really crazy, you could peel a layer off the 24mm tube or replace it with a hand-rolled FG one and save 4-5g.
IMO, the Gyro Chaser isn't any more complex than a Rose-a-Roc. This has an internal hub, but the Rose has split blades. Burn strings are less reliable as a rule. So six of one and a half dozen of the other.