Hi Tayeb,
It's the first firings that are critical, I'm firing e.max at 784'c for the first dentine firing on my Zubler. If you get porosity in the early stages it really is pretty much impossible fix in the later firings, often the small porosities only really show themselves after glazing. For cercon kiss I fire about 30'c over the recommended firings which seems like a lot but has improved the look of the ceramic immensely.
As for substituting e.max ceram for something else I think the firing temps of Noritake czr are too high, I've heard of the GC ceramic being used but haven't tried it myself, oh and don't let Mark Jackson hear that your even thinking of mixing and matching systems!
Even firing test tabs only gets you so far, I think probably due to the amount of ceramic being fired and also the position in the chamber and what the tab is on (plat foil, ceramic pillow, honeycomb etc) A better test would be to build a few units on metal/zr/emax or whatever entirely in Transparent and put them on whichever crown stands you usually use and then see how homogenous the ceramic looks.