The stats alone are not enough to understand how a material will behave, important but not enough. You have other things to look at: shader complexity view, which is also good, but again not precise, and the best one is the profiler. This one will give to you the entire scene analysis and you can dig by feature what is going on. Your experiment is the best, because when you multiplied the instances you would see the difference in milliseconds, which is what you want to care about. Also, multiplying the number of meshes (if they are the same) with the same material would not show much difference, because GPU is optimized to handle similar things in a very efficient way. If thou, the meshes has even small differences and the parameters for the same material differs also a little, then there is not much similarity and the optimization is gone and now you will rely on brute force.
The profiler is the best tool always. The stats can’t tell properly because sometimes they have built-in conditions which makes the flow of instructions being executed differ, and since I do program in HLSL a lot, I can say that a material with 1 thousand instructions showing in stats might run in a pass depending on conditions only 200 instructions sometimes, in others materials with 100 instructions have a loop which will run 1000 times for the same pixel !!.. thats why you can’t rely only with stat info in my opinion.