[Teutone]

Personally I would leave everything as it is with the idea of doing something to what you've got .
I've got a Sept. 1991 built , sold in 1992 560 SEC . Didn't want to add anything to it that wasn't what came from the factory , with an exception :
I first changed the oil over to pure synthetic ( 0-40 ) then put a K & N air filter in the housing , 3 prong spark plugs of the highest quality with high quality wires , a copper pick up distributor cap / rotor then put 45 lbs in the tires (not recommended on rainy wet days ) for cruising and distance driving only . Make sure your fuel filter(s) are new and just doing these few things you will notice an increase in its response and pep . Plus better mileage .
Just something to think about .........

A member paid a renowned competition engine builder a boatload of money for Max everyday drivable power in a M117 engine. Special headers + front to back exhaust system were all done, but their flow computer simulations showed the 560 intake runners the main culprit. Looking at the 450 intake manifold showed better flow runners, they cut it open, and build / increased deck height for more flow. With some other improvements, I think they got to 370hp everyday usable power. I believe the member spent close to $20k US. There should be a good thread in R107 or W126, or both.

Several members went to Mercedes TRY-Y headers, each cyl bank goes into dual per side downpipes on either side of the car. Got rid of the kidney looking side resonators for 2 in line staggered hush puppies, a high flow 2 in, 2 out Kat (eliminates need for cross pipe), and muffler. Dyno showed a 39hp increase.

It's somewhere in this widebody rebuild thread Project Arizona, The Rise of the Phoenix...a Wide Body story