On a related note, I use the halyards to climb the mast, which I plan to be doing a lot of while replacing the standing rigging. Don't think I'd feel comfortable ascending anything smaller than about 10 mm, though 8 mm halyards sound attractive in other respects.
The break strength of our 5/16 halyards is about 7K pounds. That is as strong as our ss shrouds, approx.
All of the loads on our rigs are spec'd out for a multiple of the expected everyday loading, to provide a safe reserve.
I am confident that our halyards will always be strong enough to hoist my 200# up the mast, but others are certainly entitled to an even higher safety margin.
Note that ability of the line to run thru the sheaves and cheeks is actually an earlier priority in the 'decision tree' to determine this sizing.
Edit: rereading my reply, it may sound like I am really "selling" the idea of the 5/16 or 8mm line sizing. Worth mentioning is that I did try two new 3/8 " ($$) spinnaker halyards, and after feeling the friction/drag thru the cheeks at the mast head, I sold them on CraigsList, replacing them with the 5/16 size. They were not a happy fit in our clutches, either.
There was no practical difference that I could really see in 'handling'. So, yeah, bigger always seems like it ought to be better, but anything above a strength-minimum is really unneeded. And do check your clutches and self trailers for compatibility, too.