The issue sounds like the network connection. An 8 Mbit connection works out to 1 Mbyte per second. Assuming that you could get all 8 Mbit for your move, you're look at 7.5 minutes to move 450 GB of data. Realistically you'll probably take closer to 10 minutes because of signalling overhead, retransmits, etc. 3+ hours ends up being less than 2.5 Mbytes of data per minute being transferred. Something's not right there.
I'm sure that your Exchange traffic isn't the only thing on the WAN link at the time, but unless the link is incredibly congested I would expect to still get much higher transfer rates. Have your networking guy look into it. If you can't get the network situation resolved, then you'll have to either live with the long move time or try something else.
The something else that I would recommend would be to bring the new server back to your site, plug it in, move the mailboxes, then ship it out to the remote location.
One other method (which is much less preferable) is to use EXMERGE to export the mailboxes to disk and remove them from the current server. Then burn it to disk (or a USB hard disk), ship/take the disk to the remote site, and then use EXMERGE to import them on the new server. After that you will have to manually reconnect the user accounts to the mailboxes. Also, you will lose your single-instance storage for the messages that were exported with EXMERGE. And it will break recurring appointments on users calendars.
