Huh, just now noticed the lenght of 17cm gun - the barrel would have to be over 12 meters long, and I doubt it can be used in anything smaller than Ratte
For sabot on the new guns, you may try giving them APDS ammo, ie longer sabot ranges - Germans had some experience with APDS for long-range artillery and due to tungsten shortage tried to go that way also for AP projectiles, namely there was some 7.5cm design by the end of the war.