Yes. Key size has no impact on the save files. All MODX models have 128 notes and same buttons/controllers/features. The difference is in the number of piano keys you get presented in front of you and the mechanics/materials of that keybed.

Therefore, a backup file on any MODX, regardless of the number of keys, can be restored on any other MODX in the world.

What you would want to watch out for is the firmware version on each keyboard. The "best" thing to do - particularly if these are both your keyboards - is first ensure both are updated to the latest version. This is because if the MODX8 say has a later firmware - then the Performances you have created may be using new features not available in the MODX6 with older firmware. Before doing a firmware update on the MODX8 (if applicable) - you would want to perform the backup BEFORE the upgrade. This is because firmware upgrades have a good chance of clearing your memory or otherwise invalidating your saved data. It is always recommended to save data you want to keep before a firmware update. Assuming the MODX6 doesn't currently have anything of value you do not necessarily have to back it up. Again, this depends if it has anything you wish to keep or not. At the end, it will receive the transplanted MODX8 data.