Ok, my mistake.
Apparently centos 6.3 cannot boot from xfs.
In theory one could probably still do it, as the prerequisites are given.
- xfs needs at least a grub 0.97 as a bootloader XFS FAQ - XFS.org
- And centos has this version: ftp://centos.bio.lmu.de/centos/6.3/o...el6.x86_64.rpm
Lots of overhead though :(