Yet to see any use a different hammer spring for DA than for SA so exactly how would the hammer impact the transfer bar/firing pin with any amount of differentiating force that would even be negligible?
The difference isn't in the spring, it's in the different amount that the spring is compressed in SA vs. in DA.
Experimentation/observation will show that the hammer drops from farther back (more spring compression) in SA than in DA. Usually it's not difficult to see the difference. That would indicate more spring compression which means a harder strike.
One can also construct an experiment to observe the difference in the strength of the firing pin strike by using an object such as a short section of brass rod that will easily fit down the bore but is fairly close to bore diameter. You want it to be big enough that the firing pin will hit it when it's in the chamber but not so large that it binds or catches on anything.
Cock the hammer and drop the rod down the bore. When it's against the breechface, point the revolver upwards and pull the trigger. Measure the height that the rod jumps out of the bore. Do this several times and take the average height.
Now repeat the test, but this time, pull the DA trigger until the cylinder locks in place and hold it there. Then drop the rod down the bore and finish the trigger pull. Repeat this several times and take the average height.
Compare the average heights.
With my GP100, there's very little difference but I have one DA/SA autopistol where the rod jumps nearly twice as high with the SA pull as the DA pull.
I want to re-emphasize the word "theoretical" in my previous post. There's a difference in how hard the hammer hits the firing pin and it's measurable in some cases, but IMO it's not a practical concern--just an interesting factoid.