I don't think the high tensile steels are necesarry for the kind of structure one finds in a touring bike. On the other hand, more conventional tube sizings made in high tensile steel wouldn't be a bad thing. Realism has to come into it though. Finding a top maker of touring bikes who has ample extreme steel experience in the real world you are contemplating would not probably be the easiest find.

Ti in bikes is no longer regarded by most as the ideal material it once was. There is at least one guy on the touring forum who has been riding a BG Ti touring bike for decades and really likes it. One thing though, if you think lugs, butted tubes or in-field repairability are top priorities, it probably isn't the best material for you.

I don't know what you are thinking about lugs vs. welds, but most of these materials are designed to be welded in the first place. And these materials started being welded back before the first world war, bikes included. Lugs are totaly proven, and if they add a lot of beauty and satisfaction to your ride, then go for it, but it isn't a durability issue, and I am not sure it ever was. It is difficult to weld very light tubbing and maintain perfect alignment. But modern methods have totaly solved that problem. TIG has it's aesthetic and structural advantages, they just aren't as well understood. If your perfect touring bike includes classic geometry then you will have the choice of either method, if you end up with materials like Ti, Al and odd geometries, you will be better off with TIG. To me brazing is more about the molded joint look, I see little advantage to it on the main joints, it is high heat, and weight relative to TIG, well proven though. All bikes will probably have some brazing on them it is a very important secondary process for nearly all builders, if one can even consider stuff like drops secondary.