Whether the car body is down or not doesn't matter. What matters is the force pressing down the wheel. In this specific case the rear wheels.
So the weight is shifting to the front when braking. There is nothing you can do about this that does not decrease deceleration. What position the body is in is not primarily concerned with rear tire contact except:
- If you allow the body to tilt forward you need faster spring expansion in the rear. Theoretically that still wouldn't matter, except that you only have limited amounts of maximum spring expansion rate.
- A change in suspension height also means a change in camber. Since the 911 has a simple struct in front and a multi-link suspension in the rear [1] that can get quite complicated. Camber adjustment with the car at rest doesn't matter much, it would be more useful to look at camber under heavy breaking.
So the weight shifts to the front, tilting the body to a certain amount, and what you want is game the various subsystems in the suspension to keep pressure up on the rear wheels:
- You want fast expansion of the dampers in the rear so that the wheel is kept on the road. Ideally the range and speed of expansion would be so big that you could ignore all the other stuff - but it is not.
- If all of this is happening while the car is already being turned in you want a stiff anti-roll bar in the rear so that the inner rear wheel can help more.
- You want slow compression of the front dampers to keep the body upright if, and only if, this actually helps due to running out of rear expansion.
The reason why you can't just dial it in with maximum fast expansion in the rear and minimum compression in the front is that there are drawbacks. Imagine you hit a curb. The slow compression in the front will make the car jump very hard. Later, when the rear wheel is past the highest point of the curb it will instantly expand its suspension, while the wheel is in the air. After your flying lesson the car lands (hopefully[2]), with the front being kinda neutralish expanded and the rear expanded a lot, only wheel on the side that hit the curb. Now the body tilts heavily away from the curb, which is usually the opposite of what you want, not to mention you induced impulse away from the curb. So if you can get away with it don't slow down compression of the front too much in an attempt to keep the body upright. First adjust the rear rates and only then check whether messing with the front is still needed and if so has a large benefit to make good for the general disadvantages of stiff suspensions.
An upright body under breaking means a flying body off a curb.
[1] the Boxster/Cayman for example has simple struts on all wheels, which is why the rear wheels look so funny on photos of fast cornering. Most other higher end modern sportcars have multi-link all around which reduces the complexity of camber changes on suspension compression. (ETA: of course the multi-link suspension means more unsprung mass, which makes all the fancy suspension mechanics less effective)
[2] this gets more complicated if you take different aero into account. If the car does not have a rear wing, but relies heavily on a rear diffuser you really don't want to fly. The diffuser instantly becomes useless on lift. This just isn't worth it except maybe for single hot laps.
