Chad
Are you doing server or client driven migrations? Either way the likely bottleneck will be the copy operation of PST data to the EV server. Once on the server I have seen 4-5GB per hour. One thing to remember is that PST migrations are a case of start, monitor, monitor, monitor, troubleshoot, monitor, wait and then pray everything goes smoothly. Don't box yourself into a corner by promising it will be done in a given time frame. Setup a pilot group that is representative of your org, track the migration process from this group and then extrapolate to the wider user base.