In this letter we report on 400Gb/s polarization multiplexed 16-state quadrature amplitude modulation (PM-16QAM) transponder variants and flex-grid network upgrade configurations. We address transponder sub-carrier granularity, and demonstrate that the performance improvement, from dual-carrier to quad-carrier super-channel configuration, is limited to ~1.4dB (in Q-factor, at power spectral density of 10-1mW/GHz), at the cost of doubled hardware requirements. In view of that, we establish the performance improvements, for a dual-carrier 400Gb/s PM-16QAM transceiver, as a function of increasing forward error correction overhead (FEC-OH) and spectral inversion based super-channel fiber nonlinearity compensation (SNLC-SI). We show that increasing the FEC-OH improves the transmission performance, at the cost of significant power consumption requirements, alternatively, employing SNLC-SI, at a lower FEC-OH, is a more power efficient solution. Specifically, for homogeneous and heterogeneous launch power based network configurations, SNLC-SI enables ~23% and ~45% reach improvements at maximum considered FEC-OH (45%). At a fixed distance, it enables ~25% and ~50% power savings, respectively, compared to FEC-OH employing linear compensation only.