I wonder how to rearm the German battlecruisers with 17in guns?

I realized that I would want to rearm the German battlecruisers with 17in guns, not just the battleships. That raises the question if ships like the Derfflinger could be armed with 8-17in guns, or would the armament have to be 6-17in guns? Could the ships have 8-17in guns and give up some speed? We could add bulges to increase the beam. That would almost certainly be necessary, due to the great force from firing a broadside. Perhaps they become 26 knot ships, rather htan