The TW523 has an opto-coupler interface that is usually operated in current mode. Also, there are no signals that need to be fast. Together, this means that the TW523 is "capable" of running on very long cables, maybe a few thousand feet.
But the detrmining factor will be the other end. The TW523 interface can run on signals from 3 volts to 20 volts. Most devices interface to it with either 5 volts or 12 volts. 12 volts would have high noise-immunity, especially if operated in current mode. The cable should not have much effect on the signal.
Do you know what the TW523 interface on the Stargate looks like? If you have a schematic, I could evaluate it, but I believe that you shouldn't have a problem with 100 feet of Cat5.
BTW: I am assuming the XTB-II is using the same interface circuit as the TW523, or if it is not the same, I assume it is better.