Hi Ramesh,
If your requirement is to have authorizations based on the PSA (assuming Personnel Sub Area) then I believe using the org key is indeed the best approach.
As far as your math is concerned (175 PSA's times 6 roles), it would only make sense to create 1050 separate roles if you have 1050 or more HR administrators. If from an organizational point of view certain administrators are always responsible for more than one PSA then I wouldn't bother creating one role for each PSA. You could save yourself a lot of effort by combining those PSA's into the same role.
If you really do have 6 administrators for each PSA then perhaps structural authorizations isn't such a bad idea. If you can come up with a function module that retrieves the persons from the PSA that the user is responsible for (eg if all users belong to the PSA they are responsible for then you can just get all persons belonging to that PSA) then you can get away with having only 6 roles and let the structural authorizations take care of the PSA (and therefore also PA) part. That would save you a lot of time in build, maintenance and provisioning.
Good luck,
Brent