Quest GroupWise Migrator for Exchange PO Growth Average?

Has anyone used Quest’s GroupWise Migrator for Exchange? Is there an industry-standard figure for how much growth I can expect due to attachments becoming uncoupled from their message chains? We’re going to use this product for a GW 7.0 to Exchange 2007 migration later this year and I’d like a ballpark figure. I know every PO and user is different, but I’m hoping for an “average guestimate” on what to expect.