Ah right, yes I see what you're referring to now.
I think he's using a lower ratio gear set and was concerned that lightening the whole power train would result in the RoF going TOO high and causing pre-engagement. You're correct in what you're saying that if you're trying to produce an all out high speed build then heavy parts will increase wear slightly but it depends on how you get there. If it's a low powered spring with a high speed motor then yes I'd agree but in this case he was using a high torque motor to get the same result so the weight of a standard piston and aluminium head wouldn't make so much difference because of the extra power available from the motor. There's a number of ways of skinning this particular cat though!
As far as wear goes, high speed builds will wear quite quickly anyway so the minimal additional wear from a heavier piston isn't going to make much difference if the whole thing is lubed and shimmed properly.
How did it turn out Ian?