We just finished a project that had a distibuted video system in it that one of the panasonic plasmas was about 200' of wire from the XP8 at the head end and the RS232 worked fine. We had another location about 300' from the head end but ended up putting in a PCM8 so we never tested RS232 that far.

As others have mentioned, the spec just says it will work at least 50' and that is up to 115.2K baud. Most devices we are controlling are 19.2K baud and under which aids in reliability over long distances.