We study the impact of the shape of optical filters on the bit error rate (BER) of a multi-channel spectrum-sliced wavelength division multiplexed (SS-WDM) system, incorporating nonlinear semiconductor optical amplifiers (SOA) to suppress the excess intensity noise, using a parallel implementation of Multicanonical Monte Carlo (MMC) algorithm. Both the slicing filter (SF) at the transmitter, and the channel selecting filter (CSF) at the receiver are allowed to be independently chosen from a reduced set of filter shapes. BER curves corresponding to a fixed SF bandwidth and bit-rate are traced for various bandwidths of the CSF. Both the single channel case, where the only impairment is the "filtering effect", and the multi-channel case, with crosstalk, are considered. We estimate the achievable spectral efficiency of SOA-assisted SS-WDM passive optical networks (PON) when both SOA-based intensity noise suppression, and forward error correction (FEC) are employed.