I have used the C Links for many years. One slightly different procedure, is that I put J B weld (not quick set) between the two C links just before I join then and peen the little protuberances over. I also apply a dab of the JB weld on the peen, to cover any galvanizing which may have been destroyed.
I don't have any scientific tests on the SWL, or braking strength. But they have always held, including in some high load conditions. (Let me say that under those conditions, the c link was on a secondary anchor,and my primary anchor was original full chain, with no weld etc.
I would not weld G4 chain. The steel has been heat treaded in the process, and you will loose the temper. Also if welded, the welder should use the same composition welding rod as in the connecting link. I don't know if a certified welder would guarantee his weld under these conditions…After heating you will loose the galvanizing--and thus need to spray with liquid zinc paint, or some other coating.
The original article stated:
" Fortunately, most gypsies
can cope with a single link that is bigger than the chain links. During research for this article I discovered that my own anchor chain was
joined using a larger C-link. It had been in use with two windlasses for many years."
I have used C links in several gypsies in 3/8, 5/16 and 1/4" G4 Chain, and generally it as worked OK, with only the single link slightly mismatched. .