Dear Buki,
There is no fixed single trend in the performance in optical communication systems. The system overall performance depends on the specs of many components and the interaction of signals in the channel. Typically, you expect increasing the bit rate to decrease the performance and reduces the Q-factor. However, the system frequency response could be better matched to a higher rate. That will cause the performance to improve. The key is you need to optimize the parameters for your rate, then characterize the system for range/OSNR, etc…
I hope this helped.
Regards,
Ahmad