We present high-speed transmission results using two of the most promising silicon photonic (SiP) modulators towards 400 Gbps per single wavelength in intensity modulation/direct detection (IM/DD) systems. A GeSi electro-absorption modulator (EAM) enables net data rates beyond 300 Gb/s, with successful transmission of 160 Gbaud PAM4 over 100 meters of single-mode fiber, achieving bit error rates (BER) below the 6.25% overhead hard-decision forward error correction (HD-FEC) threshold of 4.5×103. Similarly, a SiP differential-drive C-band traveling-wave Mach-Zehnder modulator (TW-MZM) supports 175 Gbaud and 160 Gbaud PAM4, also meeting the HD-FEC requirement after 100 meters of SMF transmission. Results on 256 Gbaud OOK are also included for comparison purposes. While further improvements are needed, particularly in reducing optical losses, managing self-heating, and improving driver integration, these modulators represent leading candidates for pushing IM/DD links toward the 400 Gbps per wavelength target in future short-reach optical interconnects.