Sure, It is very nice to increase the constellation level to increase the data rate or the transmission rate. However, as you are increasing the constellation level, you will use much power and hence the non-linearity will appear in long haul.
DP-QPSK is the most common used one.
DQPSK is used to ease the detection without LO.
I think Q = sqrt (SNR). So Q^2 is a proportional to SNR and can be measured in dB. However, you can measure any value in dB, it is a log scale even if it is a unitless like SNR (Unitless).
If you are using eye diagram, the Q factor is estimated from the Eye opening. So, it may take very large values in case the Eye is well opened.
There is a BER test set that generate a stream of data and receive it where it compares the generated with the received.
In case of using DSP to compensate the dispersion, Eye diagram will not give you reasonable results.