In case it helps anyone: I just updated a few more mega2560, and I noted the USB IDs.
If you have succeeded in entering DFU mode, lsusb will report
03eb:2ff7 Atmel Corp
(Note you do not need to unplug and reconnect to enter DFU, indeed doing so will revert to normal IDs !)
Once flashed, unplugged and reconnected, the mega2560 is
2341:0010
Earlier messages suggest that the Uno is
2341:0001
HTH 