This week I finally assembled and tested the router based on this mobo (inside case from an old Asus RS100), with an additional 6x1G NIC called Silicom PEG6 and 2x1G+2x10G combo NIC called IBM EN0T. Everything seems fine, pushing 10G through two additional fiber ports consumes nearly 1% of CPU (since almost everything is offloaded), so theoretically it could handle about 180G duplex speed across all its ports in L2 mode.
So everything seems fine except these X540 NICs do not support speeds between 1G and 10G. I only found out this today, after I purchased some 2.5G USB3 NICs to run over my existing Cat.5e cables...
Anyway I'm happy to have this little puppy, since its price was almost like any usual old mobo, but it has some advantages.