I used 5/16 for my new halyards
Josh,Last summer, I replaced my original halyards with 5/16 inch Yale Ultra Ultra Low Stretch. I went down a 16th of an inch because I got a deal on a couple of discontinued small size Lewmar rope clutches which I mounted on my cabin roof where I could reach them without straying too far from the tiller. 5/16 is the largest size of line which would fit through the clutches. My O'Day22 is hull number 3048 built in 1981 and I am sure the quality and strength of lines today is much superior to those of 20 years ago, so I'm not at all concerned with a failure. The only concern that occurs to me is that the halyard might jump the sheave at the masthead when raising or lowering and get wedged, however I'm advised by all and sundry that there is little risk of that happening when moving down from 3/8 to 5/16. I haven't replaced the sheets yet, that'll have to wait another year. Good luck.