The only benefit I can think of to adding more weight would be the increase in stiffness that has been mentioned. I don't know how big an issue this is, but my gut feel is that even the lightest plane allowed by the rules would be stiff enough that the extra stiffness you could add at the expense of a few ounces would not help you.

It is not true that added weight has no effect on absolute speed. In straight & level flight, the plane still needs to generate enough lift to support its weight. A heavier plane must generate more lift. Induced drag increases as the generated lift increases ( actually, it increases as the coefficient of lift increases ). In fast, straight, level flight, the coefficient of lift is very small, so the induced drag is also a very small fraction of total drag. Thus, between turns, this effect would probably be very small. One thing that might make this effect bigger is the habit of flying the whole course partially banked. A plane flying straight & level in a bank will need to fly at a higher coefficient of lift than one flying with its wings level, so the weight would be more important. The biggest effect of extra weight is in the turns. In a pylon turn, the coefficient of lift is fairly high, and induced drag becomes a significant, maybe even dominant, component of total drag.

I'm guessing that a bunch of extra, or unduly dense, material in the empennage would be the most wasteful. Since pylon planes likely have the CG located pretty close to neutral, the tailfeathers should not be too heavily loaded, and the fuselage behind the wing need not be extra heavy duty. If flexing is going to have adverse effects anywhere, I would guess it would be in the wing. In front of the wing, of course, is a heavy, furiously vibrating engine, which requires a nice stiff structure there.

banktoturn