I've been tethering a x51v through a Cingular RaZr for about a year now.
Recently upgraded to the EDGE / 3G capable RaZr v3xx.
While setting up the bluetooth tether link to the new phone I discovered that the x51v Bluetooth Modem settings max out at 115kbps. You'll notice this is in-line with everyone's quoted bandwidth.
Unless there's other modem software for the Axim that allows alternative settings, there's no way to reap the higher bandwidth via EDGE or 3G. Of course, the hardware may be limited to 115kbps regardless.
It seems this thread has some success reaching 1MBbps:
http://www.aximsite.com/boards/showthread.php?t=122247
Ideally it would be nice to have a Bluetooth Networking Service that acts more like a Network Adapter to your phone. Win XP SP2's native bluetooth service actually offers this on a PC but the phone has to have the service too. Of course, the v3xx doesn't.