So raised an MS ..In summary, no impact is expected is MG is enabled during mailbox seeding, or for mailboxes already in-sync.
The main points to note are:
Will a mailbox being seeded continue to seed if Multi-Geo is enabled: Yes – Seeding will Continue
Will performance of the seeding be affected if Multi-Geo is enabled: No – Performance will not be impacted
When Multi-Geo is enabled, will a user be requested to restart Outlook once moved: Not expected, it is transparent to the end-user as uses Autodiscover in conjunction with the PDL value