I am confident that more power can be arranged quite easily by Gen3Benz. But it is probably best to just get it running and find out the weak links and get everything dialed in with 400. 400 HP is a really good number in a 201, 550 is way too much in my opinion. 385 PS in the AMG hammer w124 does enough damage as it is.

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Current Stable:
1994 S500 v140, 210k miles, white with grey.

Former Mercedes in the Stable:
1983 300CD Turbo diesel 515k mi sold (rumor has it, that it has 750k miles on it now)
1984 300CD Turbo Diesel 150 k mi sold
1982 300D Turbo Diesel 225 sold
1987 300D Turbo Diesel 255k mi sold
1988 300 CE AMG Hammer 15k mi sold
1986 "300E" Amg Hammer 88k mi sold (it was really a 200, not even an E (124.020)
1992 500E 156k mi sold
etc.