Data transfer needs to be 100% reliable - the board will control cnc machines so any error is likely to cause damage or even injury
Judging by the amount of data you seem to need to move, you're rolling your own protocol - G-code just isn't that bulky (unless maybe you're doing high res 3D contours or something, and even then 115k can drip-feed with no problems). Sounds like you need to add some error detection and handshaking.
The baud rate errors are just facts of life, as you've gleaned from the datasheet. Additionally, some arduinos (the USB models, IIRC) had a USB interface with a baud rate that was not quite right, so the arduino core was mangled to match. This can cause issues communicating at some speeds (56k, IIRC).
As best I can tell, you can either roll your own arduino with a crystal chosen to match your RS232 needs, or maybe use an external UART like this one http://www.strichlabs.com/pages/multiserial-shield that matches speeds better.
In any event, if you're shoving data around at 230kbaud, you're going to have to watch your coding to ensure you don't drop any.
-j