Having owned 631, 853, Platinum OX, and a bike that is a mix of 853 and OX, I would say that 853 and OX are equivalent. They are both superior in ride to the lower grades. They feel more lively and at the same time less flexy. I guess the best way to say it is the 853 and OX both flex when they are supposed to and are stiff when they need to be. The lower grades don't do this as well. Columbus also has a 853 grade, but I don't know what it is.

As for 953 (stainless), I've been investigating the Independent Fabrication offering and they tell me the build issues mostly stem from the fact that the material is so hard.