[Quickurt]

To answer some of your concerns.
I asked the dealer exactly the same questions you are asking and most of it depends on how low you want to go.
For the 20mm I questioned on a 987 the factory accessory springs are well within the parameters of the stock shocks.
The 30mm lower springs are enough stiffer that new shocks are suggested, however, I was told several customers had gone that route without changing shocks and were not complaining.
Anything lower and coilovers are suggested.
I was told on both 20 mm and 30 mm lowering, the stock 987 suspension allowed enough adjustment to maintain proper alignment, albeit barely on the 30mm. Anything lower also required replacement/modified parts for alignment.
I do not know and did not ask about 986.
With the ways I use my car I am leaning heavily toward doing the 20 mm lowering.
Maybe wifey won't notice............