I have not seen any difference between using z2 alone or topping with CS. I did both of my cars several months ago and have been monitoring them wash to wash. They are both red. I did my 2006 330i with zaio,z5,z5,z2 and my 2002 325xi with zaio,z5,z5,z2,cs. As an experiment I also did my 1997 328i (hellrot) with zaio,cs. They only difference I've noticed is the 97 is looking like water is not beading as well on it after about 3 months. This may be somewhat of an unfair comparison since it spends a little more time out in the sun than the other 2.
Ultimately, my conclusion so far is the CS isn't worth the money I spent on it. That opinion may change over the next few months if I start noticing a difference but by that time I'll be doing my winter prep so it probably won't matter. I may run this same experiment through the winter since we get some major salt put down here.