You are so right Tim, why redesign what they have provided? Moving the rack or changing it's body length (distance between the inner tie rod end pivots) is moving it out of the arch and pivot points of the control arm and will provide bump steer.
Moving the rack doesn't effect ackermann at all. The only thing that effects it is the location of the tie rod end pivot point in relation to the knuckle or spindle pivot point in relation to the center of the rear axle.
This is a good example of how moving the rack or puttinging the wrong one in effects things.
If the racks tie rods are not matching the control arm they make a different arch. This different arch in effect "lengthens" and "shortens" the rod. Look at the drawing, when the wheel goes up the arch of the end of the tie rod goes in, turning that wheel in!
When the wheel goes down, it's pivot is out further, effectively "lengthening" it turning the wheel out!
The control arm pivots and thus arch has to match.