The actual shock valving is the big key here, and that may be what you are feeling. I think it is in agreement here, that a 50/50 bump/rebound rate, for the most part, works very well. And that is the tough part, as most shocks do not specify that rate. I had my fronts custom valved by Bilstein, and my rears..., they are what I needed. Too slow a rate in the rear, will cause the car to jack down, which causes evil handling, as the shock does not let the suspension springs to actually work. And yes, the shock if too stiff will give you a tendancy to either over/understeer, depending on which end is too stiff. Koni shocks were really bad for this...I been there.
You don't want to use the shock to set the over/understeer characteristics.
Bumpy..jerky...that gets to be a ride preference thing, balanced with the reality of what is the car really doing...on a given road surface. Things are always a trade off somewhere
No matter what, under whatever condition, things are kinda right, if the wheels are staying on the road, IE the car does not feel skippy on the surface. You need to feel that the tire..and of course the feed back to both your ass, and your hands on the steering wheel, is staying in intimate contact with the road surface, under all but the most severe conditions. Again, say under very hard braking on a short frequency bumpy surface, you may feel the tires almost locking, under the brakes.Suspension "stiffness", should not be confused with lack of CONTROLLED suspension travel. As some of us use very high spring rates, that is still matched to a shock that can control the whole package so to speak, AND, the resultant weight transfer, both fore and aft, and, of course body roll during cornering.
The other thing to not forget, as we all throw money at all this trick suspension stuff, is how hard are you using the car?! If you are just farting around, you may find the ride is going to feel stiff, with all the stuff you have bought, from reading this site. But, really get the car up to speed, and I mean speed, and not straight, and that is when you really do find out how things are working. What feels stiff now, is sure going to start to feel a lot softer as both the g-load and speeds go seriously up. And that is said with the fact that the anti -roll bars will effect how things work, as much as anything else. As some of you know, a bunch of us run NO rear bar, as our rear spring/shock rate, plus our 1 1/8" front bar, gives us a slight terminal understeer condition at the limit, but..below that the car is totally drivable with the throttle. And never forget....WHAT IS YOUR DRIVING STYLE?
Personally, as the speed gets up there, I have never found a oversteering car very fun to drive. At the slalom, or other controlled area, fine. But, in our real world of canyon runs, scenic tours, bumps, gravel on corners, yada, yada., you kinda need that thing to not want to come around on you, should you have to get out of the throttle, on the brakes, in a corner, blah, blah. Again, get the car reasonably balanced, with the throttle to control the over/understeer tendancy, and you will have a fun package, that will keep you safe.
With todays adjustable shocks, not that the valving is perfect, you can at least run the thing at a reasonabale ride, and try to use the spring rates to control the handling aspects, plus tire pressure, plus wheel spacing, can all fine tune things.
All things considered, my car does not really ride that harsh, at least according to some of my passengers, but then I have lived with the things for 30 years in this mode. I actually have my GTS set up the same as the Datsun, and as it is a heavier car, could actually be a tad stiffer in the spring/ant-roll department. If you are the type of driver that really toss'es a car in, this requires that the aforementioned spring/anti-roll bars/camber, etc, become even more important, as the instantaneous g-load can really ramp up there. I measured my 510 once at almost 2 g's on a quick toss in.
I to cannot comment on the rear springs, as wheel rate needs to be figured. You are on the right track, for you, and, as you have already seen, there is no one magic setup for all conditions. It is all about comprimise, you just have to figure out which one you want to sacrifice, for the type of driving you want to do the most.
Hope that may help some.
EDIT: as i thought about this some more, what we are talking about here is the spring frequency, which is what we feel in our seat of the pants. A soft spring has a low frequency, so we just wallow along. As the spring rate goes up, that frequency goes up, so we end up with a ride that feels "jerkier". That is the trade off, for both better roll control, and weight transfer under braking. When I thought back to my earlier days of experimenting with springs and shocks, ride quality became secondary to roll control. No brainer there, but, with our 510's, as most of us have found, we have needed to run quite STIFF, so as to minimze the rear toe change, which is the worst of the handling quirks. In using shocks built for heavier cars, we do have to be careful that it's bump /rebound rate and the speed with which it can react, is close to the stiffer spring rates we use. Other wise, it will just plain be too harsh for our needs, unless we are really pushing the cars suspension.
I know in the case of my GTS, I really went for the stiffest antiroll bar I could get, and just upped the spring/shock rate a reasonable amount. I went with 300# springs up front, and Tokico adjustables. I can soften the shocks up for rain and snow conditions. I live in this thing everyday, it needs to be compliant in rainy conditions ( and snow). The rear I left maybe a tad too soft, but, that solid axel and four link I neeed to be ok on some gravel roads that I go on. Again, a comprimise.
not to be confused with someone on a soap box 
). Now going down to 2 I noticed the car was much more spongy on the freeway and I felt slightly less confident with the car, so maybe I should turn it up one click at a time and drive it for a week on each number to get past the initial shock of change in ride and really get a good assessment of the change.
