Not possible by firmware cross-flashing, at least without very heavy under-the-hood firmware manipulation and even then probably not feasible either. The key to being able to flash one unit's fw to another related unit using either complete [original] or hybrid flash is in the donor and receiving units having the same PCB P/Ns: [Only registered and activated users can see links. ].
nuvi 2595 has PCB P/Ns of 105-01925-00, 105-01925-02, 105-01926-00, 105-01926-02, 105-01926-10, 105-01916-00 & 105-02067-00 whereas zumo 660 has 105-01452-00, 105-01452-01 & 105-01452-02 and zumo 665 has 105-01452-01 & 105-01452-02. No matches sorry.

I think from memory the only other hardware related to 25x5 is nuvi 150.