Well, it probably doesn't help you much, so let's wait for an answer from Michael,
but i can say that i had no problems to extrude all objects at once, with my little
iMac 4GB of RAM under OSX 10.6.8 with the 2.5 OSX beta. Boolean calculation takes
on my Machine forever, so i stopped that task. MoI didn't hang, just for the record.