I understand your concern but if you think about it, B&M make this shifter to suit almost all cars, so they're pretty confident you can make it work. If the factory bracket on the auto only lets the cable travel 15mm and the B&M travels 10mm (using your example) you just lower the point that the cable attaches to the linkage on the trans to minimise the radius of the circle and thus, less travel (10mm).
I had a Pro Ratchet shifter in one of my VL Turbos and its was wicked for quick shifts while battling the wheel to keep the car straight. Not sure if I'd like it in a street car though.