One thing to bear in mind Alex is that drums generally have a lower kJ figure than most spinning discs or blades. This is due, in part, to the fact that most drums tend to be much smaller compared to discs. Kenny's disc on 180 is 5 (or 127mm), but you'd never see a 5 drum on a beetleweight as it would most likely be too big for the design and too heavy (even Drumroll's drum isn't even that large!). A drum's weight is also spread out over a wide distance whereas a disc is much thinner in comparison, so it can put that weight saving into a heavier rim which, in turn, creates more inertia when spinning and therefore has a much greater output kJ figure.