So I am a little put off by those in this forum who are saying it's fine to overload a horse. Please publish proof before you encourage someone into danger and injury for both horse and rider.
I don't think that anyone is saying it is fine to overload a horse, the discussion is always around what the actual fair load is. I of course have a vested interest, I will state that, being an overweight rider, and I totally get what you say about centre of balance and other factors, I know that I am a better rider when I am lighter. I also promote any size of rider increasing their fitness, again my son is 6' 4" tall, weighs 250 pounds of mostly muscle, and I would let him ride my little Haffy with no issue, because he could support his weight and so could she.
Again, the study that you quote was looking at horses being ridden all day at trot and canter, and it makes sense to me that if you are going to be doing that then you need to maybe look at the weight ratio differently from an hour walking on the flat.
I don't know where you have suddenly popped up from, but this is a continuing discussion on this forum, and many others, and the only conclusion I have drawn is that when it comes to weight carrying the only answer is "It all depends"
There is no one formula or one single study that will be able to take into consideration the vast array of shapes, sizes, ability and fitness level of a rider, with the same variables in the horse. Now add the extra variables of tack, terrain, length of ride and the exertion level required, and you would have a formula many pages long I feel.
I still struggle with the notion that 20% is fine to go and do what the hell you like, (according to some people) but at 23% I am 'overloading my horse' when all I want to do is walk and trot him 2 or 3 times a week for half an hour to an hour. Is it better that I just let him sit, and the day I hit 20% I can just get on him and go for a gallop, because hey I'm now reaching the magic formula.
So yes, an argument without end, yes there have to be limits, and most of them are based in common sense.