A great question for your setup shop who is doing the corner balance and alignment. It will vary based on your all up weight, tire choice, dampeners, sways, and performance driving experience. No one simple "best" answer sorry.
Here is a brief description of the loads and forces a pro setup shop evaluates before choosing the proper springs. As you can see it's just not so simple as "pick this one!". Dynamic Load Transfer
Yes, my car has lowering springs that are somewhat stiffer and about 20mm lower than stock. I have no idea which ones they chose during my car setup as they plugged all the variables into a spreadsheet to generate a few possible choices. My car is a street/track Time Trial car that competes about 20 track days/year. My setup shop has a long history of putting their customer cars on the podium and I trust their judgement. Race track setup is not a DIY job if you want the best results IMO.
If you just use lowering springs, you lose some of the benefit of the dampers. OTOH, you've lowered the center of gravity, so it's pretty much a wash. My car has H&R lowering springs that are an upgrade from stock and I like them a lot, but this is the sort of thing about which an HPDE driver can worry himself to death.
X73 is sublime and just works with 20 mm of lowering. So many others can be choppy, bouncy, and just not as refined as the Porsche OEM. If you add GT3 control arms or eccentric bushings and push the car's track out 6-13 mm more it will also lower the car a further 3 to 6 mm getting close to the 30 mm of lowering on the GT4. The balance at the track is perfect.
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