This GL550 (GL500 overseas) has the following modifications:
RENNtech 200cell downpipes
Custom flap exhaust
Custom TCU and ECU remap
Basically a turbo swap and Renntech tuning to make it work. The result is impressive:
588 ECE corrected whp at a correction factor 1.009 in 28°C ambient temp. In other words, just under 700 crank horsepower which they could easily hit but Renntech believes the automatic transmission can not take it.
Renntech states the torque converter slips at ~950 nm or 700 lb-ft of torque at the crank. They dial the torque down for longevity on this car.
Clearly, tuners can do this M157 swap so why aren't more people doing it?
677 hp at crank
865 nm / 638 ft lb torque