Regards how much weight a mast will take - a lot of masts will virtualy bend into a U shape especialy rdm masts.
430 with a 38cm extension - you will probably find that your sail has a much softer feel, opening up and spilling wind easier/earlier. Sometimes there can be issues with the foot batten nor rotation well due to it meeting the mast right at the bottom where it is stiff. Also be carefull about which extension you choose as they are more liable to bend/break when set that long.
460 mast in your 6.0 - opposite effect to above the sail will feel tight and more powerfull. by raising the sail you also raise the center of effort and center of gravity, niether being ideal.