Why are you using RS232 with USB in the first place?
MCUs don't speak RS232, and modern computers all have USB, so what is RS232 doing in the mix at all? FTDI makes plenty of USB to UART adapters that run up into the double digit Mbps, will that not work for you?
If it's a long distance run you're worried about, then go with something like 422 or 485 instead.