Has anyone tried this yet and if so, opinions and diameter?
I changed all my halyards to 5/16" Dynema. Plenty strong enough. They work great. I have internal halyards with different exit boxes so you will have to check the sheave on your boat. If it is too small for 5/16" you can strip the cover. I'm sure someone else has done this before and can tell you if the sheaves will work without stripping the covers.