We report on compact and efficient silicon-organic hybrid (SOH) Mach-Zehnder modulators (MZM) with low phase-shifter insertion loss of 0.7 dB. The 280 µm-long phase shifters feature a π-voltage-length product of 0.41 Vmm and a loss-efficiency product as small as aUπL = 1.0 VdB. The device performance is demonstrated in a data transmission experiment, where we generate on-off-keying (OOK) and four-level pulse-amplitude modulation (PAM4) signals at symbol rates of 100 GBd, resulting in line rates of up to 200 Gbit/s. Bit error ratios are below the threshold for hard-decision forward error correction (HD-FEC) with 7% coding overhead, leading to net data rates of 187 Gbit/s. This is the highest PAM4 data rate ever achieved for a sub-1 mm silicon photonic MZM.