I have a '92 W124 Factory Sportline. I changed front and rear shocks to Bilstein Sports. The Factory Sportline shocks may be Bilstein but they are not the same as the Bilstein Sports. Compression is slightly stiffer on the Bilstein Sports.

I also had a '93 Factory Sportline at the same time. Therefore, I was able to
confirm that the additional stiffness was not simply due to worn original shocks on the '92. The '93 was smoother than the '92 with the Bilsteins.

My opinion is that the Factory shocks are a better compromise between handling and stiffness. If you want slightly more stiffness get the Bilsteins.
I don't think it will improve the handling much.

Steve