You did not pay too much for it.
The thing you should look at - hard - is the condition of the stock head. To get all the workings in the action into the head of that stock, the cheeks are left pretty thin. My drilling, of similar design, had a disconcerting hollow sound when one tapped on the cheeks. I had a gunsmith remedy that.
The minimal wood in the stock head then and it's age (approx. 100 years now) militate in turn that you stick to low-recoil loads. Putting 2 3/4 through 2 1/2, regardless of the pressure issues, will increase your recoil. And that won't do your stock any good. As you note, there are already some cracks showing, which tells me someone in this gun's past didn't heed my advice. I don't see pressures being an issue b/c the barrels on this gun are likely stout. It's made by Germans....
BTW, it's regulated for a 13 gram steel-jacketed bullet. That's the "StMG"; 13 grams is 200.97 grains. Forget about steel jackets. You can get, or cast, 200 grain bullets in all the 8 mm calibers you want. With cast, you can assemble nice mellow loads and shoot all day.
Congratulations, and welcome to wonderful world of drillings.
