Thought I' toss my $0.02 in here.
I have the W&D street springs in my '83 Spider, along with the W&D rear sway bar, on 14" 195/65 Dunlop Sport A2s. Handling is very good, but the ride is stiff -- not intolerably so, but stiffer than stock. Ride height is about 1" lower. I use Koni reds -- settings about 1/2 on front and full soft on rear. Alan indicated he prefers Bilsteins, but he didn't throw up when I told him that I'd likely keep the Konis since they were nearly new. Personally, I think the Spider doesn't benefit from any additional boost in spring rate in the rear, so I'll stick with Koni reds.
The debate over whether to use stiffer springs or sway bars to control body roll is a long-standing one. The sway bar has the advantage of pemitting softer springs on the straightaway and therefore more ride comfort, since sway bars only come into play when the suspension is loaded in turns. Each suspension tuner tunes to his preference, which may not be yours, so you should try to drive cars that have been set up with the systems you're considering. When I was redoing the rear suspension in my '83, I corresponded with a number of folks who raced Alfa 105 coupes in Europeand their consensus was that stiff front, soft rear was the fast setup, including disconnecting the rear sway bar. Again, these were race cars, so ride comfort wasn't an issue, so "soft" rears needs to be kept in context.
The Spider's basic problem from a handling perspective (compared to the GTV or Berlina) is that the rear is relatively light and the engine does not have gobs of power to plant the back end a la Corvette. Rear springs that are too tall exacerbate the problem of getting weight transfer to the rear to keep it planted. The GTV and Berlina are somewhat diifferent, one because their respective wheelbases are longer, two, they have greater suspension travel (particularly in the rear), and three, they have more weight in the rear. Because of that, sweeping gneralizations about all the 105/115 cars should be consdired carefully -- the generalization will be right (it is, after all, the same basic suspension and drivetrain), but at the margin, the devil's in the details, so don't assume that the set up from a hot 105 coupe is the right set up for a 115 Spider. And it's best to remember that we're talking about performance at the margins here -- the stock suspenson set up on a new Spider makes for a very good handling street car right out of the box. The problem now is that very few of us have driven a *new* 115 Spider as a frame of reference. Comparing a clapped out stock suspension to one with new go-fast parts is a somewhat misleading exercise, to say the least. In fact, for a street car that doesn't see much track time, I'd advocate the stock setup with new stock springs (and urethane bushings and a rebuilt steering idler). The stock setup is more car than most of us are drivers (and I include myself in that statement), plus it will be a very enjoyable ride on the street.
Ironically one of the best things I ever did for handling on the Spider was to put roll bar in it. Stiffening the body twist helps a great deal and I'm told that a roll bar with the brace from the roll bar down into the passenger footwell is more effective than the chassis stiffener and legal in the autocross/racing classes to boot. The other thing that really tweaked the handling was to put a trailer hitch on my Spider -- hanging the extra weight out there pulled my corner weights to within 3 pounds of each other on the diagonals, so now the car *is* just about perfectly balanced, and that's another legal modification.
YMMV.