Yeah, you can get quite a distance even at 115200 baud, this is not like an RF frequency.
I forgot to say my setup uses 0v-5v serial BACK to the PC, so it's +/-12v RS232 one way and 0-5v back the other way. No problems at all with the 0-5v serial comms at 115200 baud and 40 feet of stereo coax.
CATx cable using a twisted "pair" would probably work even better still.
Re the cable termination I do the opposite; put a series resistor before the cable (at the transmitter end). That gives you a little bit of RC and smooths the transistions, and asynch serial handles sloping transitions really well (but can have a problem with ringing if your transitions are too fast). From memory my existing setup uses a 220 or 270 ohm series resistor on the transmitter side(s), which is also good insurance if you get a cable short.