The forum at
www.mbcoupes.com is performance oriented towards the SEC.
One or two people have super modified their cars. I believe cyberdrakon, (Eddie) had a NOS system in his 560 SEC (I saw the car in 2002 in Las Vegas) and someone else had a turbo.
This company has converted them, for the W126 cars and I have heard they
are still in buisiness. It's an expensive proposition though
http://www.mosselmanturbo.com/
The owner of the website at mbcoupes owns a shop, and it's mentioned on his site. He could advise you. His personal car is a beautiful '89 car, that he has put in a Mercedes V12, after first putting in a V8 M119 motor. His applications are very reliable and complete.
http://www.youtube.com/watch?v=QpytnbbMfyw
He's the one person I would be willing to leave my car with, throw money at it, and say: "Make Nice"
Great guy with a great reputation, Satish Tummala knows and loves the 560SEC. His shop is in Michigan.
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1991 560 SEC AMG, 199k <---- 300 hp 10:1 ECE euro HV ...
1995 E 420, 170k "The Red Plum" (sold)
2015 BMW 535i xdrive awd
Stage 1 DINAN, 6k, <----364 hp
1967 Mercury Cougar, 49k
2013 Jaguar XF, 20k <----340 hp Supercharged, All Wheel Drive
(sold)