Here I go again with my opinions. At least these are opinions that are solicited.
124 cars are EXTREMELY DIFFICULT to beat. However, they're not small block Chevy's. I'm a hot rodder from way back, and have ported more heads and changed more cams, intakes, carburetors, headers, etc. than you would believe. But these aren't things to do to a Benz.
There are several reasons for this: Performance parts for these cars are EXPENSIVE. These engines already have very good volumetric efficiency. There are several reasons for this good volumetric efficiency. The main reason is that in Germany, cars are taxed(heavily) based on engine displacement, therefore these engines are designed to wring out as much horsepower per volume as they can get and still have a drivable, reliable, smooth running car.
In contrast in the US, Detroit suffered very little penalty back in the muscle car days for providing LARGE engine displacement. In those days exhaust and intake systems were restrictive, and cam timing was so conservative it could be called extreme right wing. For this reason, cams, intakes, headers, etc gave extemely good results, there sas alot of room for improvement. On the Benz engines there is not near so much room for improvement without it being expensive and making the car less driveable.
If you want more power than a later twin cam, six cylinder 300E, consider one of the V8's, 420E or 500E.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in