stock and aluminum spacers? As I said on an earlier post, I've done the grey silicone mod, and I'd guess the stiffness about 30% of the way from stock to aluminum (that is, closer to stock than to the aluminum). I should say here that my stock mounts were not leaking or showing signs of going out. I'm with Gordo: I'd guess your mod is closer to the aluminum, but will wait for your analysis.