I have Koni's and have tried a couple of different spring rates and lengths on the front and read and I have experience with using the Kori threaded collar kit, as well as, the 928MS spring adapter kit.
I used H&R springs instead of Hypercoil. Both manufacturers make the same liner spring in different weights and lengths.
Weights
My car is a GTS so it's a bit heavier than early cars. I started out with 1000 lb springs in the front and 600 lb springs in the rear. I used the Koni's at the maximum dampening. This setup was really good for smooth roads and track driving. The ride quality was smooth and not harsh. The car would take a "set" very predictably on the track. However. I found that on uneven surfaces like back roads and forest roads, the car felt over-sprung and the Koni's were not able to deal with repeating bumps. The rebound was too fast and there was not much compression over stuff like speed bumps. I ran these springs for 1 year.
I decreased the spring rates by 20% to 800 lb in the front and 450 lb in the rear and have been using them for about 6-months now. I find this combination to be much better for off-track driving, bumpy undulating roads with lots of twists and turns and ups and down. I would definitely recommend these weights. I would not say they are track only. My impression is that they are stiffer than new Boge Red spot shock but offer much better compression control than the Boges where sometimes I would bottom out my nose on dips in the road (as stock ride height). I was just out driving last night on 50-miles of Pacific Northwest forest roads which follow rivers and climb up and over 1000-ft hillsides. Roads are uneven sunken grades and pavement quality varies since maintenance is inconsistent. These shocks are not too harsh for this. My GTS is setup for sport driving, not urban cruising and daily driving. If this is the case I might forgo some performance and go with 600/350. BTW, this car originally came with Bilsteins and those were very comfortable for city driving, but vague when trying to initiate a "set" in a turn.
Lengths
As for spring lengths. You want to make sure you don't go too short such that the spring coils with "Stack". Some people like shorter springs because they are lighter. This is more of an issue in the front than the rear because the front shock travel is very short.
REAR: 12" will be fine. I am using a 12" spring in the rear and the Koni top spring perch and that length seems to be just fine. I definitively would not go shorter.
FRONT: 8" is a good place to start. I started out with a 6" spring with the 1000'lb springs but then changed to an 8" spring with 600lb.
With the 600/450 lb springs, I had to reduce the dampers to 50% of the adjustment range and the car seems very happy. I have not fussed wit the damping since installing them. But I am very happy so far and would definitively recommend this setup for a car that is dedicated to fun driving and road trips. Urban city driving with lots of speed bumps can be a bit annoying. But my 928 deals with those better than my 2018 Golf R.
Hopefully you found this useful. If you have any questions, hit me up.