The performance increase should be substantial (depending on the drive, of course). USB maxes out at about 20-25MB/s. With eSATA you should get nearly the "native" performance of the attached drive. 100MB/s should be no problem.
You can find some eSATA numbers at http://plugcomputer.org/plugforum/index.php?topic=1783.0
for a RAID1 enclosure.