I don't see why you would need to write your own driver. CDC class can use the Windows generic driver already available with Windows.

Microchip & PBP both offer example programs for CDC class USB communications.

As for data rates, download & review the USB 2.0 & CDC class documents from http://www.usb.org These explain in detail what data rates you can expect depending on the USB transfer type you're using.

You might also want to take a peek at Microchips' AN956; Migrating Applications to USB from RS-232 UART with Minimal Impact on PC Software.

Any route you take is going to involve some serious study time. USB is pretty complex.