Jason,
I haven't looked at the thread you mention in a while, but if you take springs that are exactly the same in every aspect except the height, because they have different number of coils, then they will have a different spring rate. The spring with more "active" coils will have a lower rate (softer), and the spring with less "active" coils will have a higher rate (stiffer).
Think of it this way. You have a very tall spring (many coils), and you push down on it. Between each coil, there is an amount of space that you will be able to compress by applying the force. With a lot of coils, there are a lot of little areas that you can compress. If you were to compress an exact same spring, but one that has less coils, there would be less number of little areas that would compress. (I hope this makes some sense...

)
The Ward and Deane "street (sport)" and "race" springs do share the same material, wire diameter, mean spring diameter, etc., but they don't have the same number of coils. The "race" springs have less coils, are shorter and therefore, also stiffer.
Alan Ward from Ward and Deane racing is still around and he still sells his suspensions. (If you would like Alan's number, please send me a PM.) When I took of my old W&D "street (sport)" springs and placed them next to the new set, they were the same. I must also mention the following. The sport springs that Centerline sells have the same specs that the W&D have. They are yellow in color.
I must also say that there are several combinations that work well. For example, after many years of resisting ('cause sport springs lowered the car too much) a good friend of mine finally installed IAP springs with Bilstein shocks. He came around in a roundabout way. He found an incredible deal on Bilstein shocks, which he loves in his other cars. At the price he got he figured that if he didn't like him he could still re-sell them and actually make money. (It really was an incredible price.) Well, since he had the shocks he figured it was time to replace the springs on his Spider anyway, so he made a couple of calls. The only supplier that stated that the Bilsteins would work with their springs was IAP. (Centerline correctly told him that their springs are too soft for the Bilsteins.) He took a chance on the IAP springs and it do lower the car. He took the car out for a ride, and he didn't quite like how they felt. They were stiff. After a week of on/off driving the springs, shocks, etc. settled and felt great. He mentioned that even while lower the car never bottoms out. Every time I speak with him he tells me how happy he's with his new found suspension and how much more they are taking the Spider out on short and long trips. He kicks himslef for not having done this sooner, as many of the roads we encounter in the NJ and NY area are not very good, and stock Spiders bottom out and bounce horribly giving the feeling that they are going to fall apart when going over a bump.
Best regards,