Funnily enough, the topic of conical pistons came up over at Dave Owen's only yesterday, while looking at a couple of dismantled ED Racers. A while ago, Dave did a bit of experimentation with a conical piston in one of his T2.5 prototypes, and it made no discernable difference to either performance or handling. In fact, the only effect was to needlessly complicate the manufacturing process, so he went no further with it. Steve Rothwell has apparently come to the same conclusion.