Bonzo Banana, I think you're missing the one crucial fact that has been pointed out to you already; that high carbon steel is a VERY broad category, and encompasses many different variations in alloys, tensile strengths, and fatigue limits.

It's hard to figure out exactly what/where you are trying to go with this post. It almost comes across as if you think that the older Hi-Ten steels are better and you are looking for definitive information to unseat that opinion, and you're sort of wandering around the information given. If that's the case, there is lots of information available on the web about different steel recipes and what they are used for. You may want to brush up on this information so that you can better tailor the questions you ask for the information you need. I'm not saying this to be crass or rude, but you'll be able to gather more information in a shorter period of time than you will with a question and answer technique on a forum. Some more general knowledge/information will help you in getting the answers you are looking for.