As a satisfied beta user who transferred my group from Yahoo in January, I can say that I think your transfer process will go flawlessly. And in the odd event that any flaw does happen to occur, you can be assured that Mark will take care of it immediately.
My group is much smaller than yours but still has 400-600 messages a month), and some very large groups here (larger than yours) have transferred without a hitch. Go for it.
Messages are the sole opinion of the author.
I wish I could shut up, but I can't, and I won't. - Desmond Tutu