I think that all Thorn frames are a frame for life - At least I hope they all are!
953 has a much greater Ultimate Tensile Strength than 853 2000Mpa - 1400Mpa. Given that the density of each material is about the same this means that 953 has a much better Strength to weight ratio than 853 - It is 'allegedly' a better ratio than the best Aluminium or Titanium grades.
There are some drawbacks though. To make use of it better strength to weight will require thinner tubes. Thinner tubes are more susceptible to damage. Another issue is that 953 tubes are rolled and welded, 853 is extruded, all things being equal an extruded tube is more robust as there is no weld to fail. The other big draw back is price.
Having said all of that I would love to get my hands on a Cyclosportif like bike made of 953, it would be a rocket.