Moving to MySQL was fairly easy, don't recall exactly what changes I made for that (probably only changing the JPA config file). In the schema, at one location there was an embedded object instead of a reference to that object, I don't recall where that was, but it was really easy to spot (I think I saw it once the schema was loaded into MySQL and I used the MySQL tools to browse the schema) and easy to fix (added an annotation, if I recall correctly).
I used a JMeter script to populate the users. And another script to populate the auction items (with a really long auction time - SeamBay does really weird things if you bring up the server and the auctions have expired - it pretty much screws up the database and you have to start over again with a clean database and repopulate it).
Then I wrote a JMeter script to put the whole system under load. I documented that script on a white paper that I presented at the CMG 2008 Conference. I would provide a URL to the white paper, but only the white papers thru 2007 are available to the public - you have to be a CMG member to get at the 2008 white papers. The 2008 papers should be made public sometime next year.