Performance would be somewhat worse.
You did not mention how many drives you have hooked up (or make and model),
but with U320 SCSI, you still have the advantages of better command queueing and
device disconnect features. Giving the SCSI drives better I/O performance, especially
under server use conditions.
If you were using a controller with better support for Ultra320, then there are a few
other features you could take advantage of, but as it stands, the current controller
you have has less bandwidth than an ATA-133 setup, but better I/O.
IMO, speed wise, something like a couple of Raptors would have better data transfer
for its files, but the SCSI drives would seem more responsive to user requests.
And there are "enterprise" class IDE drives that are designed (and rated) for longer use.