For a test, I put a 45 foot cord on mine and it seems to be working fine. I had a 6 conductor wire, not normal 4 conductor (Does that make it RJ25?), but I doubt that affects the outcome since extra 2 wires are just disconnected.
I get the same readings before and after adding the additional cable and the temperature readings seem to stay stable.
It does, however appear that disconnecting the temperature probe with the unit running stops the BrewBit from uploading data to the web. It recovers after power cycling.