I agree, the rigging will be sloppy loose if 1/4" wire is tensioned to 267 lbs (3.8% of breaking actually).
Also, if you are planning on upsizing the wire, remember to upsize everything else from chainplates to turnbuckles to mast tangs to match otherwise it is a waste - and regardless the load on the boat is greater.
Wire seldom has a load problem - mostly it is a corrosion issue. Larger wire will corrode as fast as smaller wire.
I do not plan to increase my wire size (5/16inch) but just trying to get an understanding of the dynamics of the rig tension- Thanks.
One thing I find interesting is that it seems sailboat rigs seem so fickel, when all around us are power poles and guyed antennas that get almost no inspections and still stay up. Seems they are using galvanized wire. Do large antennas need to have their guys replaced evey so many years? Some of these antennas are huge and the wind load must cycle them just like a sailboat rig. The below tower is 1000 feet tall and has an elevator to get to the top- located in Baltimore MD.