I've finally figured out why I am able to get 12.0 Mbps USB modem port
speed with the V3xx tethered to one Windows XP computer with DUN
(dial-up networking), but only 921600 bps on another similar Windows XP
computer.

When setting up a new Windows XP installation I was able to get 12.0
Mbps port speed when tethering the computer to my V3xx with the Motorola
Handset USB Driver you have available for download. I was still able to
get 12.0 Mbps port speed after installing Motorola Phone Tools version
3.11, as well as after updating it to version 4.3.6c (the latest version
that doesn't exhibit the USB driver crash I reported in Reference Number
070709-000012). All well and good.

However, after pairing the V3xx with that same computer, which installed
it as a Bluetooth Modem, the maximum port speed for the Motorola USB
Modem dropped to 921600 bps, the same maximum as the Bluetooth Modem.
Apparently, pairing the phone with Bluetooth changes the Windows XP
configuration in a way that prevents the higher 12.0 Mbps port speed for
the Motorola USB Modem.

I tried uninstalling the Bluetooth modem, and deleting the V3xx from
Bluetooth, but that did not restore the 12.0 Mbps port speed.

I then tried uninstalling and reinstalling the Motorola USB Modem, but
that did not restore the 12.0 Mbps port speed either.

I had to use System Restore to roll back the Windows XP system to a
point prior to Bluetooth pairing of the V3xx, whereupon the 12.0 Mbps
port speed was restored for the Motorola USB Modem.

--
Best regards, MOTOROLA WIKI:
John Navas <http://cell.wikia.com/wiki/Motorola>