I asked because if you had ever had aftermarket tubing exhaust headers, you would know the downsides to them. These negatives are; they are prone to leaks, they rust out over time (of course the stainless ones mentioned above obviously would not rust), they are somewhat noisy, and because they are an aftermarket item they usually provide obstructions to access of other engine components such as starters and the like. Also with many modern engine compartments, which are crammed in almost all cars these days, it's necessary to pull the motor, or at least raise it, to get them in place.
In my younger hot rodding days, headers were a great improvement for any Detroit V8. Almost every one of the Detroit engines came with EXTREMELY restrictive log type manifolds. Most aftermarket headers in those days were made with flanges that were too thin and leaked constantly. These leaks will occasionally warp an exhaust valve, if it happened to stop almost closed at the leaking header port. But in spite of all that, the improvement was worth the hassle.
On every Mercedes I've ever seen, the cast iron exhaust manifolds are impressive pieces. They always are layed out such that you can tell by looking at them that they flow well, not just a log with some openings toward the exhaust ports. I expect that the main reason for this is the fact that in Germany, cars are taxed(heavily) based on engine displacement. This means that M-B (and every other MFG in Germany) has to build cars that make the most power with the least displacement. Detroit can build horsepower with cubic inches instead of engineering efficiency into their standard motors. This of course has gotten better in the last decade or two.
I expect that the headers on your 190 were factory stainless items, well thought out for that car, and properly built. I wouldn't want to bet that I could get exhaust headers that good from the aftermarket.
So to wrap up my thesis here, I believe that you have very little and maybe even nothing noticeable to gain in return for a lot of hassle and potential maintenance problems in the future.
It's easy for me to make a lecture like this toward someone else, but at the same time, I understand the urge to make improvements.
That's my $0.02, I'm sorry it took you $100.00 worth of time to read all of it.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
Over 800,000 miles in