@kopca
In my uncertain recollection, your nuvi 2757 doesn't have a BT module nor a microphone so ASR isn't available to it either. A mic could be retrofitted as there would likely be an empty socket for it's connection or at least solder pads on the board. The related senior model 2797, one of which i own, does however have both a BT module and mic. I've previously partly-converted my 2797 to 760 using nuvi boot.bin and retaining original HWID without issues but cleared NV before and after each flash. It's presently properly functioning as a 2797 and has been for some time.

You should always clear NV before and after loading a non-standard firmware. That's not optional for me, it's essential imo. A byte or two of incompatible data remaining in the non-volatile region may eventually cause an irreversible bricking later. Certainly clearing NV doesn't guarantee against that possibility, but does greatly reduce the risk for sure.