Just a quick update, if anyone is interested.
After long consideration I decided to tackle the problem myself. And now, after many hours of carefully grinding down the markings in the chamber by hand, many hours at the reloading bench, and many trips to the range I finally had some real progress.
Today I squeezed just above 2200 feet/s out of that old rifle with a 270 grain RN bullet. Using a slower burning powder seemed to help keep pressure down.
I still have intermittent problems with the bolt sticking a bit, but nowhere near the way it was in the beginning when I first wrote this post. I could probably squeeze a bit more out of it, but I don't feel it's necessary. If everything goes as planned, this Saturday it will be my short-range/backup rifle on a boar hunt. This will be the first time this rifle is used for hunting for almost three decades.
Thank you everyone for your input, this forum is invaluable when it comes to Mannlicher Schönauer info, and it's thanks to you.