Timber frame construction method impact sound insulation upper floor

I suspected that as well and it already gets me a bit further!

Does the mineral wool insulation have any other unbeatable advantages that mean I should definitely use it?

Otherwise, we save the money and invest it elsewhere in the house.

clad with 12.5 GKF.

How is the GKF attached to the joist layer? If the ceiling is double-planked on the underside and the whole thing is well decoupled from the joist layer (tongue and groove rail or suspended), you can improve impact sound insulation a bit, depending on the floor structure above and the adjoining components.

There are, for example, special sound-absorbing stone wool insulation from Rockwool. We have a comparison at our place: part of the upper floor is insulated with Styrofoam (hallway, bedroom, bathroom) and the two children's rooms with Rockwool. The flooring is the same laminate except for the bathroom. The difference, whether the boys scrape their toy cars on the floor in their rooms or in the hallway/bedroom, is noticeable. In general, stone wool provides better sound insulation than Styrofoam, probably because it has more mass.