What *exactly* is the cost of Xenos AA?

the Z-pass tag (for tiling) each polygone?
that must use a little bandwidth for write tag for each poly into RAM?

 

No, the Z pass tags a command - this will usually be many polygons.

 

ok but if each individual poly is not tagged then they will are more poly consider into several tile (and tranformed every time) and if the tag is by "objet/coherent-group/primitive" the use of small poly in the next-gen game will not reduce the relative redundancy of geometry process

 

All depends on the average size of your objects/command blocks.

Yes as I've mentioned before there will be some duplicate vertex work, and frankly minimising this is really what requires you to plan for tiling in the first place. You know what's going to happen so you need to strcture your data to minmimise the impact.
There are tradeoffs to bne made in batch size vs screen coverage and vertex shader complexity. Tradeoffs you really need to benchmark to make good decisions.

Doesn't take a genious to do this, but you pretty much have to plan for it.