Is there an issue anywhere in trying to get 3MBps as a lot of USB->Serial adapters and devices use this speed.
Have you tried faster rates? No software issue but it's up to the hardware to support it.
We've had requests for this in the past and the node-serialport code has already been modified to not restrict the maximum speed, so it's up to the hardware and the OS serial port libraries how fast you can go.
Reopening as a docs issue.