Hi Andi,
If all the geometry of the boat/rig remained the same as a standard boat and somehow, magic, you increased the righting moment 15%, all loads in the system would go up by 15%. This not to say that all loads go up by the same number of Kgs, but all loads go up by the same percentage. For example, the axial load coming down the mast is a much greater force than the sail force, but if the sail force goes up by 15%, so does the mast axial load, compression, coming down the mast.
In your situation you have it better than that. The increased sail force is due to increased righting moment. The majority of that increase in righting moment is in the trapeze wire which intersects the mast at the most shallow angle of any wire connected to the mast. Therefore the increase in mast axial load and main beam bending load will be less than a 15% increase. Other loads that add to the total mast axial load are windward shroud tension, which could go down, forestay tension and mainsail leech tension, mainsheet tension. In a simplified analysis we could say that if the horrizontal sail force goes up by 15%, these other forces will also go up by 15% of their base value and we are on the safe side.
As far as your main beam goes: Uniform composite parts, beams, loaded in bending fail on the compression side first. Therefore it is important to add material to the top of the beam, under the mast step, and tapered as the additional laminate flows away from the center of the beam. Axial strands of carbon is all you need. The base beam can handle any axial cracking loads.
Good Sailing,
Bill