I often use an External USB to RS232 Converter to control products with RS232 connections and I haven't noticed any problems. I use the same comms routines (using WriteFile and ReadFile) as when it is connected via RS232 directly. I can't comment on the baud rate as I use a fixed rate but I have not noticed any problems with retries.
I have the comms set to 1 stop bit, no flow control, not overlapped * baud 38400. These are the timeout settings that I use:-
CommTimeOuts.ReadIntervalTimeout=50;
CommTimeOuts.ReadTotalTimeoutMultiplier=1;
CommTimeOuts.ReadTotalTimeoutConstant=50;
CommTimeOuts.WriteTotalTimeoutMultiplier=1;
CommTimeOuts.WriteTotalTimeoutConstant=1;
Hope that helps :)