The Columbia 26 Mk II is listed at a displacement of 5900, so it starts out 1500 lbs. heavier than the Columbia T26.
As I said above, I don't believe that the displacements on the Columbia specifications are for fully loaded for cruising boats, which will add at least 1500 lbs. or so per person. At least that is the figure that my experience has said is about average for each person.
Liveaboards are usually a bit heavier, so call it 2000 lbs. and that would put it in the ball park, especially given that the Columbia specs seem to be for the unloaded, empty, unequipped boat—so the difference will probably be about 3500 lbs, rather than 3700 lbs.
Mind you, I'm guessing based on the specifications from the website I posted above.
One other note on displacement
—according to the USCG, for documentation purposes, my boat displaces five net tons. It weighs less than two.
Many catamarans I know of have a displacement, at least according to the USCG, which is far more than they actually weigh. The actual displacement of a vessel, if you could measure it accurately, would give you the vessels weight, as it is the amount of water that is displaced by a volume of water equal to the weight of the boat—unless the boat has sunk, in which case, the displacement is not accurate.