The M156 responds well to simple reversible tunes, with owners reporting 500 to 540hp from Renntech, VRP or Weistec stage-1 flashes, often on an OBD dongle that can be removed for sale. Tunes from a former AMG engineer's firm are viewed as low-risk, though a related C63 dyno confirmed roughly 450whp on a tune.
CLK63 tuning: Renntech, VRP and Weistec to 500-540hp
Mercedes-Benz CLK55/CLK63 AMG (C209) · mechanical