Navy Six
Well Ok, a couple of thoughts. First thought, Trigger pull only measures trigger pull. A poorly fit trigger can have a heavier "pull" than a well fit trigger, regardless of Main Spring.
One would have to build a jig to hold ALL subject Main Springs in a like position, then connect them to a gauge that measures the amount of "pull" required to move the action end a measured amount of deflection. Then one would have to measure the effect of "stacking" (increased pull) over the full deflection of the spring. Once documented, one would have to insure there was absolutely NO drag in the moving parts of the subject test action.
The results, as applied to the reaction of Percussion Caps, would then be skewed by the differentiation of material used by the manufacturers of differing cap brands. Also skewed by the differing types/amounts of explosive material and thickness of sealant, also skewed by specific lot numbers of the various manufacturers.
The same variations would also be observed in various Main Springs, by manufacturer, based on lot numbers (unknown) and varying dates of manufacture.
Considering ALL the variables and complications, I think the objective "Gee, That one feels different" is as definitive as it's going to get. along with measuring the specific Brisbane of the various caps. Also, the test subjects would all have to be tested on exactly the same set of nipples. Yule also need to arrange for a High Speed High Frame Rate camera and a skilled operator with a concurrent trigger mechanism.
Have Fun!!