The roll centre is a virtual point around which the car rolls during cornering. The car itself can be seen as mass in a single point, the centre of gravity. The lateral acceleration during cornering acts on the centre of gravity and creates an outward force.


Normally the centre of gravity will be located above the roll centre and the outward force together with the distance between the centre of gravity and roll centre becomes a roll couple that makes the car roll around the roll centre.


If the centre of gravity and roll centre are in the exact same position the distance is 0, so the roll couple is also 0, which means that in this situation the car will not roll in corners. If the roll centre is higher than the centre of gravity the car would roll the other way.


But having the roll centre high is not necessarily a good thing. It makes the car twitchy and unstable (hence the complaints on the original 3dr). Also the lack of body roll makes it more difficult to feel the limits of the car.


And what is also important is that the roll centre is not a static thing. When the suspension arms move during cornering the roll centre will also move. A roll centre that moves a lot also gives a strange feeling to the driver. So I think it's best to have the roll centre in such a way that it doesn't move much. The best way for that is (on a McPherson setup) having the arms going slightly down from the cross member to the knuckles.