Sam, your point about using high bs braid is knocked on the head by the fact that a well balanced rig should always be used.
I think there are two factors to consider.
1/ The main line will have a higher than stated breaking strain, possibly it is quoted as knot strength, and being a much larger diameter it will be much tougher.
2/ The line (either hook link or main) will break when the stretch (elasticity) has reached a point where the strain on the line is as high as the breaking strain of the weakest point. Because the hook link is much shorter than the main line it reaches the end of its elasticity first.
I don't know how well I have grasped the facts or how well I have put them across, but I know (through recent experience) that if I use shortish high tec copolymer hook links I need to use higher breaking strain than main line. For example an 8lb main (.026) and a 9lb or 10lb hook link (.21 copolymer or .1 braid), a 5lb main (.21) and a 6lb 12oz hook link (.17).