My understanding is that there is a difference between stretch and creep (and I'm no expert!). Creep is the permanent elongation of the string most of which occurs when it is being shot in. You get loads of that with B50 before it settles down, but to me it seems to go on creeping with bows of any significant weight. Stretch is the elongation under tension which springs back when the tension is removed - I've measured my B50 strings before and after bracing the bow and they stretch at least an inch just when you brace the bow, which is why you make modern material strings about an inch longer. My guess is B50 stretches a whole lot more when you draw and loose the bow - one of the reasons why it is relatively slow - energy must be going into the string instead of being delivered to the arrow, but I've no way checking this. During the course of a field shoot I reckon the brace height on my B50 strings drops by as much as 3/8", which is enough for me to have to readjust the string to get a clean shot - others may be able to get away with this change.
I've tested dacron breaking strain with my bow scales multiple times and never had a strand take more than 25lbs, but I guess it depends just how you go about the test. The linen I have breaks at around 18-20lbs so comparable to B50, but it is less consistent and occasionally a section will give up at 14lbs or so. I've never tested any of the modern materials - breaking strain just isn't an issue. I think I used 10 strands for the strings I made with 452x - is that too much for a wooden bow? It will need a heck of a lot of serving if you go much thinner. Recurve shooters use something like 22 strands with this small diameter stuff.