Quote [If anything, the suction downward at the stern is helpful!]

To "drag" your transoms on any point of sail is always the slow way. There is always a better compromise. (Besides, "suction", doesn't exist as such, it is a layman’s descriptive term for the effect (in most cases) of a head of air exerting a positive equalising pressure)

Quote [But also note that boats under spinnaker achieve higher speeds then when going upwind. Wave-making drag becomes increasingly less important relatively to wetted surface area drag with increasing speeds. As such downwind legs can actually favour shorter hulled boats more then upwind legs, meaning that when a shorter hulled boat is found to be favoured on the upwind (which is not a certainty) then it is logical that under spinnaker it will be favoured even more.]

Personally I would have thought that the fact that boats going downwind go faster than they do upwind was due more to the fact that they are carrying much more sail (with their spinnaker), rather than any alteration in the effects of any drag inducements or hull lengths. IE drag coefficients become much less important to overall speed than actual sail area/efficiency (assuming that the hulls are relatively efficient to begin with) Or perhaps I missed your point?