Repeaters work at the RS-485 electrical level to amplify the signal and nullify the effects of noise and distortion on the signal. In effect, they take a signal in, amplify it, and retransmit it almost perfectly with any distortion removed.
Like any electronic device repeaters can deteriorate with age. When you are doing an integration with the S4 Open appliance this is one of the things to check for. If you find a repeater in a N2 bus this may provide a perfect place to split the bus in two in order to improve performance.
bringing both segments into separate downstream N2 ports of a S4 Open appliance (OPC-N2 Router or BACnet-N2 Router) will lower the load on each segmant, improve error handling and resiliance of the bus, and somewhat improve performance.
bringing each segment into its own S4 Open appliance will do all the above and significantly improve overall preformance. In addition, it provides a more distributed solution that is less dependent on single points of failure.
You need to do a performance vs. cost vs. reliability and availability analysis to determine what is better for your customer's situation.
In general, the more de-centralized and distributed you can make the system the higher performance it becomes.