Since I started to use eix-sync for Portage tree synchronization. I always found annoyed when eix-sync runs with emerge --sync, I used to run emerge --sync -q. I really dont need to see a long list of directories.

At first, I didnt find the solution and I still have no idea why I missed these from eix-sync -h:

-c CMD Run CMD instead of emerge --sync.
-C OPT Add OPT to the emerge --sync command (or whatever is used instead). This option can be used accumulatively.

So, the solution is clear:

eix-sync -C -q

If you need more options to be appended, run with -C '--foo --bar'. If you need to run emerge without --sync, use -c '--whatever'.

If you are lazy like me, add the following line to ~/etc/eixrc:

EIX_SYNC_OPTS="-C -q"

-C -q will be appended when you run eix-sync.