Dear Ekrem,
The BER and Q-factor that you attached are extremely good. You don’t need to improve them. Typically, for practical 10G system, the Q-factor is between 9-12dB and BER is around 1E-9.
Your system has lots of margin. You may increase the distance, increase number of users (user larger splitter) or lower the power to reduce the cost of the system, # of users and increase reach.
Regards,
Ahmad