I think as most have been more recent additions to Zoos during the era of studbooks, their breeding has for the most part been much more carefully supervised and so they are far purer.
.. but having said that there is current discussion on the Colchester UK thread about their female Siberian/Amur tiger, which is impure due to one of her ancestors having Sumatran blood.
Would anyone know if there are any plans to breed pure-Indian Tigers outside of India?,
I guess that Indian zoo's would have stud books for there Tiger breeding?.
If you look on ISIS, a number of zoos around the World are still listed as having 'Bengal' tigers, even though most, if not all of them are generic tigers. As we know, Isis just list whatever details they are sent so it reflects those zoos which still think they have Indian tigers, though I doubt if any of them at all are pure.
The problem in Indian zoos is their Tigers are probably not so rigourously managed either, even if they maintain studbooks etc. These tigers are far less likely to be impure, as they have no need to import Tigers from other countries, but they may be very inbred, particularly where there have been attempts to breed more White Tigers.
I wish a few places outside of India would make a concerted effort to get genuine 'pure' Indian tigers again but it would be a costly exercise even if some were made available for export, and with two other subspecies more readily avalaible in zoos, maybe its not a priority.