Here's my personal experience. I had a '79, tried Bilstein's.
Made the car skittish and oversteery... Took them out, put in Koni's. Ah, normalcy returned. I bought the '85 Euro, I drove it and within the first 200 yards recognized the pesky jiggly low speed/small bump ride.
Made a mental note to check...Bilstein's, of course. Took them out, and put in Koni's. Ah, back to a ride/handling compromise that makes more sense to me. Bought the '89 GT. Took out the Boge's and put in Koni's...this time converted to externally adjustable...so I could easily set them full soft on the street, and full hard at the track. Even better. Consider that many of the complaints I hear (as demonstrated on this thread) about the ride in normal street mode might be the shocks, not the springs.
As you might expect, when I prevailed about Eibach to make some performance springs (by putting in a personal appearance at their hq, and then promising to do all the testing for free, it overcame their "small market" objection.) I consulted with their engineer (Richard Jonec at the
time) and we went the way big European touring car teams do at the 'Ring...variable rate (also called "progressive rate"). Softer initially (first 10 mm of travel) then getting stiffer. Allows you to get over the vast majority of road imperfections in your street driving, and, when driven hard (past that 10 mm of travel,) get the stiffer rate you want for that. Those big teams do it with multiple "stacked" linear springs...Eibach made this a "plug 'n play," stock diameter, but with variable spacing. (We could have used tapered wire to get the variable rate, but that would be even more difficult.) Combine these with the adjustable shocks, well this design does it for me...a dual-purpose street/track car guy. Please don't assume two things. That complaints about the "ride" on the street are necessarily the springs, it could be the shocks. And also please don't assume that it will necessarily translate into faster times at the track, either. It may not. You want the suspension to take a distinct "set" in the corner, and you want that to be "tangible" to the driver. I call it the "all hooked up" feeling.
Really stiff set-ups are vague in in this respect, taking away driver confidence, etc, so I see little point in making things hugely stiff.
(Exception: unless you can use huge wings to generate down force, then you'll absolutely need stiff springs to deal with the increasing load, as speeds increase...largely inapplicable to street cars, and common in open wheel and prototype racing.) Having spent the time to do umpteen spring, shock and alignments on various 928's over two decades, and I would think that the consistent results I would speak for themselves somewhat, I thought you might enjoy a quick personal review on this topic. Y.M.M.V.
These are the Koni adjustable shock absorber part numbers:
Koni 558240-1085 + 558240-1086 (rear and front)
The Koni's are made adjustable, but there is a tolerance issue and when
mounting you'd need to calibrate them. The speed in which the rod of the shock
absorber returns is adjustable and needs to be more or less equal on both sides.
There is almost no difference when turning open, half closed, 3/4 closed.
The usable span of the absorber is best approached when you close them fully and then turn them open one quarter turn and see if the return similar. If not then adjust a little. This needs to be don on a bench. It takes a bit of practice and a test-setup to compress the dampers to get it right.
Next is mounting the dampers in the car. Last verification to see if the car
returns synchronously. Hard to tell, I know.