High power 1300nm edge emitting LED's (ELED) will be required for spectrally sliced optical transmission systems. However, wavelength shifts caused by variations in ambient conditions and drive current will cause a degradation in system performance. We show that using current technology wavelength division multiplexers the use of uncooled transmitters causes a reduction in operating margin of 50% in a four channel system and that significant performance limitations occur when operating with multi-level line codes.