LTO3 would have a compressed capacity of 800GB if it got 2:1. best bet would be around 1.4:1, but you might get more with the Exchange Information Store.
However, if you have BEWS 2010, you can deduplicate that data to disk (assuming you have enough disk space), and hence do smaller backups to tape once done. Something else to look into.
Otherwise the only other thing to do is look into buying an LTO4 drive to take care of this (800GB/1.6TB).
And you will need full installations of BEWS on each server that has a tape drive too!
Laters!