Agreed.. do you know where you will end up migrating to? I looked into FastMail a long time ago, at that time it was the best replacement for just the Gmail component with custom domain, not sure if it still is.
I'm pretty wary of any "automatic import from $previous_service" tools (having been bitten in the past)
so I ran the fastmail importer against my user's accounts, then later reconciled this against the gmail/fastmail APIs with my own code
... it was completely byte-for-byte perfect (other than "muted" conversations, which weren't imported at all, that I dealt with separately)
I was very impressed indeed