Hi Muhammad,
The more the Q-factor, the better the system performance in terms of bit error rate. In practical systems, maximum acceptable BER is 10e-9 to 10e-12 in many cases, which correspond to Q-factor of 6 to 9. If you have achieved such high values for Q-factor, you can increase you system distance or bit rate to the point where Q-factor reaches a value near the aforementioned values.
Regards