When I was a lad, I used to fit modified discs and pads (Tarox grooved), but since then I have always fitted only genuine discs and pads on any car that I owned (including that W203 and W204).
That been said, when I bought the W204 in 2017, the dealer (non-MB dealer) fitted new front discs and pads to the car just before putting it on the forecourt. So I got a car with brand new front discs and pads for which I had no say about the make and model....
On closer inspection, they were clearly not genuine MB parts, and as far as I could tell after some detective work (i.e. intensive Google search) they were probably GFS own-brand (called VETECH at the time).
I had the car for over 5 years and covered 20,000 miles in it. The brakes performed exemplary and were still good when the car was sold.
However, when the rear discs pads were due, I fitted the genuine MB item.
So I don't have a decisive conclusion.... but in any event and in spite of the positive experience with the non-genuine brakes, I am still sticking with genuine parts going forward.