The cones still do not present a theory that sounds correct to me. I do not see any data supporting the use.
Back pressure always presents a problem with overheating the head and raising intake manifold pressure.
At Bonneville, a stock Vincent will burn a hole through the piston as you are crossing the timing mark. Racers like Alp are running a large amount of unburned fuel through the engines to keep them cool for three or four miles. Running rich with nitromethane is important.
I don't think there is too much danger in running cones, even without any data. I think there is much more data on step headers being beneficial, mostly from McIntosh, who has tested step headers on Vincents. I don't see Horners or Godet doing either of these mods.
David