It is not only in Leopard 2 A5/A6. The Leopard 2 A4 FCS also fully automatically calculates your own movement over ground, turret movement and enemy movement.
From A5 on the FCS is just a little bit more accurate but more important it is a little bit faster. Only milliseconds but these could make the difference.
As I said before the Sabra is able to hold its own in defense positions. There his inferior mobility and optics are not that big problem. Your fire line should be nearer. The main problem could be a mobile defense. Being slow means that you are not able to get into a new defense position fast enough and the pushing Ts could be able to close the gab to the defenders very fast. This is always a problem for defenders because letting the enemy not get too close is a major goal.
From A5 on the FCS is just a little bit more accurate but more important it is a little bit faster. Only milliseconds but these could make the difference.
As I said before the Sabra is able to hold its own in defense positions. There his inferior mobility and optics are not that big problem. Your fire line should be nearer. The main problem could be a mobile defense. Being slow means that you are not able to get into a new defense position fast enough and the pushing Ts could be able to close the gab to the defenders very fast. This is always a problem for defenders because letting the enemy not get too close is a major goal.