Well, the gold on gold 321 supposedly (according to the technical data tab on the Omega website) weighs 212 gram. Assuming (I do not know) the alloying material for Canopus gold is mostly palladium, the resulting 18k gold density would be 17.5 g/cm^3. Pt950, if alloyed with Pd as well, should be around 20.7 gram/cm^3, so scaling just by that would result in an estimate of 250 gram for the platinum on platinum, neglecting the weight of the movement in the scaling. (If alloyed with Iridium, it would weigh 21.4/17.5*212=260 gram, again ignoring the non-precious metal content.) So 270 gram sounds plausible to me.
Somebody ought to buy one, and weigh it!