As Scarecrow says, it depends on righting moment. The mast has to support not only the two guys on the wire but also the flying hull (which for sure is heavier on the I20) via the shrould. On top of it you have to add the main sheet load (~50kg x 7 or 8) which pulls the mast down and aft and the resulting downward force of the forestays, which keeps the mast in position (a bit more than the load from the main sheet). The downhaul will add additional downforce and bending, but this is fairly comparable between the two boats.
You can pull the main sheet harder on the boat with the high righting moment (as the light boat will fly one hull earlier), hence the higher loads.

A mast fails due to compression and bending, a special buckling case. Constant material and thickness sections will fail around the spreaders. A carbon mast has most likely more material at the spreaders, hence it might fail somewhere else.

The formula given above should fair enough to estimate how much higher the loads could be. If the mast can handle this, is an other question.

Cheers,

Klaus