As I understand ARM926 is a kind of CPU standard. It can be implemented in silicone by anyone who obtains the licence. And mediatek could implement any of that ARM standards, including ARM926.
But, anyway, those nuvi 30 and nuvi 2x5W firmwares have different loading addresses (0x10032000 against 0x00060000 and 0x10040000).
So they can't *easily* replace each other. Maybe there is some hard way, but no one is going to try it.