Same eidolons in both tests? Do you mean the eido grids? If so the non illu eido would use the standard eido grid of course. Whereas the illu eido uses the MLB water eido grid.
If you mean same eidos as in the support eidos, that doesn't really matter. Whether it's illu + tiamat/jack/reiki or tiamat + tiamat/jack/reiki, my illu setup does more dmg. Pretty sure it holds true even when I use 100%s as supports.
Method of testing? I simply see the dmg numbers and compare.
As for how much, why should I run tests for you? Seems pretty lazy to expect others to do work for you for free. If you pay me at a rate of $30/hr of my time, I'll reconsider then.