Amin,
yes the greater received power that means high SNR, so better BER.
in your case many sources might be source of errors, such as nonlinearities (make sure you uncheck it from all amplifiers and components) and noise pins.
I advise you to start from simple system which has only SMF (No Nonlinearity no noise) then verify that higher power better BER.
then insert amplifiers step-by-step to catch the source of discrimination.
thanks
Mohamed Abouseif