Depends what type of aircraft you are flying and the sortie profile Chevvron - a pointy jet climbing straight into CAS, then maybe, but a pointy jet staying in the LFS, or a military prop or helo and they still use QNH after departure.
Many Mil airfields use QFE on commencement of recovery from well above the TA; changing from a TA of 3000 to 6000 would not make a jot of difference to mil operations.
However, a common TA would make a lot of sense as mentioned above.