and for the most part it (migration wizard) did ok. There are some issues, to be sure... some others on this forum did not think the migration wizard was worth it, and used imapsync. We were pretty small, and it went ok. We did have the migration wizard bomb on attachments greater than 10 meg, and if it ran into enough problems, it would bomb out - so when you restarted the wizard on the same account, you could very likely end up with duplicates (email, calendar, contacts). So you want the account to be as clean as possible.
I did have to mess around with LDAP for a bit to get to the account selection, etc. but sounds like you are already past that.
As for supporting GW6 for migration, have no idea. We did not use a mapped drive for the domain/po, but rather UNC. Also there was some question as to the format of the xml file... I used the below...
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<PostofficeIpPortInfo>
<postoffice name="cn=MY_PO,ou=GRPWISE,o=MYCO" ip="192.168.5.2" port="1677"/>
</PostofficeIpPortInfo>