I don't understand the various sentiments about front vs rear roll center heights affecting front vs rear roll stiffness. I think the two are separate issues.
The front & rear roll center heights affect the roll axis, which affects the overall roll couple (distance from the CG), and thus the overall amount of roll.
But, assuming the chassis is torsionally stiff compared to the suspension roll rate, the chassis will roll as a unit. When it rolls, the front & rear suspension roll stiffnesses determine the couple generated at the front and at the rear, which will affect the balance of the car.
Once the amount of roll has been established by the roll axis & CG, the roll center height no longer enters any of the equations, and therefore it ought not affect front/rear roll stiffness balance.
Roll centers significantly removed from the plane of the road generate lots of scrub (sideways motion of the wheel resulting from vertical motion of the suspension), which is probably bad for stability and certainly is bad for tire wear.
As has been stated, roll centers significantly removed from the plane of the road also can generate jacking loads (swing axle beetles were also famous for this problem).
From Bastow's suspension book, changing the front view angle of the semi trailing arm largely changes the ride steer (toe vs. vertical deflection) curve, with a small effect on the roll center height. Small changes in the angle cause big changes in the curve. All the various books stating why Porsche did so-and-so are just hypothesizing. There were probably 2 or 3 people that actually knew the reason, and everyone else is guessing.
Here's a swag at a ride steer curve for a +10mm change (up) in inner pickup point.
(green: inboard point is 20mm above outboard pickup)
(yellow: inboard point is 10mm above outboard pickup)
That said, I'll hypothesize some.... The RSR & 930 had wider rear tires. They are thus more sensitive to camber changes. The shorter banana arm (inner pickup moved rearward) yields more camber gain, which means 1) the rear tires end up with more favorable camber in a given corner, or 2) static alignment settings can tolerate less negative camber.