cyclegadget:
I am curious would it be difficult to change to 7-bit data? In some automotive applications I am lead to believe they use 7-bit but, don't quote me on that.
If you want 5 - 8 bits of serial data, have you considered this library?
It replaces the existing HardwareSerial and it's quite a bit faster and has configurable buffer sizes (transmit and receive).
http://arduino.cc/forum/index.php/topic,85207.0.html
Iain