[Asbjorn]

Yeah the placement of the rear spring vs the front cuts the wheel spring rate by a large factor and also limits how high you can go at the rear. The front stiffens up much faster without hurting the comfort as much. This is especially true on the smaller engine Z4s.

I ended up running stock front spring with HR rear + a large spring spacer to avoid lowering the rear of the car. With my front sway bar in its middle setting and my custom rear sway bar in its stiffest I now finally have a setup that feels balanced almost towards oversteery on track.

On the street I would still prefer the eibach rear spring + stiffest sway bar setting front. Although such a setup causes more understeer, the stiffer front sway bar makes the car feel more direct and fun to drive on the street. The softer eibach rears also give you more grip over bumps and makes the car feel more comfortable to drive. Because the seating position is where it is, you really feel the rear springs more.

On the street a softer rear + stiffer front gives you more usable grip and a car that feels more fun and comfortable to drive. Because lets face it, on the street we don't get much enjoyment from going fast into corners. However we get enjoyment from how the car reacts to steering inputs and from how much rear grip we have when accelerating.
I mean how often do you really brake hard from 120mph to 40mph and then turn in while trail braking at the limit of grip on the street? You rarely have an opportunity to do this, its not exactly safe, and it doesnt provide much excitement either. Braking hard and turning in is not nearly as fun as accelerating hard and sliding out. However at the track, braking hard and late is all you do all the time. If you can brake later and still get the car to rotate and turn in, you win lots of lap time. Thats why a track-going car needs to be balanced towards less understeer by running a stiffer rear. To summarize:

Track/time attack preference: Soft front, stiff rear = OEM-ish front, maxed out rear = what I have done. Doesnt feel particularly fun or comfortable on the street, but it offers the most overall grip and is super fast and challenging to drive at the track. The car will loose the rear end if you do something silly like lifting off in a high speed corner.

Street preference: Stiffer front and only sligthly stiffer rear = what most coilovers + aftermarket sway bar combinations offer. You keep some comfort and rear grip by not over-doing the rear-to-front spring rate ratio, while getting the sharper response and feeling that the car can handle the extra power without squatting and diving as much. The car will understeer at the limit, which is also much safer on the street.

Personally what I do is simply adjusting the front sway bar to be stiffer on the street and softer on the track. I also adjust the dampers to be a bit softer on the street. Finally I use different tires/wheels (square setup) and brake pads on the track, but thats a different story.