We investigate the impact of noise distribution on the bit error rate (BER) in a WDM system seeded by a spectrum sliced amplified spontaneous emission. The Gamma distribution of the intensity noise was considered for accurate estimation of transmission performance in a real system. The power penalty by the relative intensity noise and the extinction ratio (ER) were calculated to identify a suitable operation range. To consider the finite ER, numerical simulation was conducted. We also investigate change of the noise distribution after noise suppression and fiber transmission, and the effect on the BER by the distribution change.