I'm putting together a showreel, using very varied material in sizes and quality.

All clips that don't follow the project settings gets scaled in importing them, adding black drapes to 4:3 material to fit the 16:9 project.
Now when I want to rescale this material so it's cropped rather than black bordered, using the Pan & zoom effect the scale ratio is set to 100% for this already rescaled material which means that I can't get it to it's original size and thus getting very degraded quality in these clips. - 720x576 material gets scaled to 540x432 for 720x432project. Scaling this material to original size would mean a scale factor of 1,333333... (576/432) which is not possible in Kdenlive, you have to use 134% resulting in a slight rescale from original size, resulting in bad quality.

Would it be possible to set initial scale value in pan&zoom to the actual scale value, 0,75 in the example above, so you can set a value of 100 to get the correct size?
Even better, would it be possible to import without rescaling material at all?

I think you want to use Crop instead of Pan & Zoom. Crop occurs before any scaling. Maybe you want to crop ~72 from the top and bottom. If you do not want to scale the source over 100%, you can try to Composite it over black. In Composite, the rectangle widget is a bounding box and by default the clip on top only scales down from 100% to fix the box - you have to click the Fill checkbox to make it scale over 100% and fill the bounding box.

Yes, you're right, crop did the trick.
I feel a bit ashamed so please ignore my rant here.
As for composite over black - I don't seem to have any transitions in my Kdenlive, all greyed out...