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I've played with front and rear roll centers, anti-squat, and camber curves quite a lot on the race cars over the years (SCCA early on, then nascar for two decades). Here's what I know.
Shortening a control arm will give a more aggressive camber curve. Think of an SLA suspension and visualize a super short upper control arm length, then watch the camber change during jounce/compression. (Assuming of course that the instant center isn't outside the tire where the camber curve would be opposite of what we want.) We would generally want a longer instant center than a shorter one, all else being equal. It's more predictable for the driver.
Raising the rear pivot points will increase anti-squat. Often this is necessary to get back what lowering the car took away. IRS cars can never get as much as we'd like because of the force vector going through the center of the axle instead of the tire contact patch as is the case with a live rear axle. *BUT* the same forces that make the rear tires "hook" also REDUCE the rear grip when you get off the throttle. (Think of 930's and the "DON'T LIFT IN THE CORNER!" speech that the first time drivers get). With brake floaters you could eliminate this phenomenon entirely, and actually plant the rear tires harder on braking, but good luck fitting that in anything but a live axle car.
Raising the rear roll center is necessary as you increase the rear wheel width. FWIW, it roughly coincides with the width of the rim (i.e., start with the width of the rear wheel, and if the rules suddenly changed to allow you a 2" wider rear wheel, increase the rear roll center about that much).
Rear bump steer. Yup, just as important as front bump steer, only harder to easily change. Wanna turn a great driver into one who shakes and sweats every time he enters a corner? Put some wicked bump steer in the rear then check the stains on the driver seat after a main event.
When we'd build a car from scratch, we'd agree on F&R roll centers and alter the pickup points to maintain those numbers. You're limited by rule on things like frame height, suspension types, spindles, etc. Those became the parameters from which we worked.
Walt and Scott are right. There is no free lunch when it comes to suspensions. The factory did things the way they did for a reason. Have a good plan in place if you want to deviate widely from their knowledge. That said, we would NEVER construct a modern race car with a strut front end and banana arm rear end unless it was required by rule. I can see why someone advised "put the rear suspension off of a 996 in there."
I'm old school. Even though I've used many suspension analyzers, I like measuring stuff. Get down on the ground with a tape and measure everything, then see where your pickup points are at various body roll and ride heights. Transfer those to the garage floor by snapping chalk lines and measure the actual instant centers and roll centers. Make changes and take copious notes. Learn how to check and adjust bump steer, both front and rear. Test on the track and see how things work. Take tire temps and pressures every single time the car comes off the track. See what changes make YOU faster. Have fun.
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