Got my new Lacie Firewire 400 pci today, and it seems to behave a bit better..
But 5.1.2 has the same issues a i had with my built in fw, i guess its because it reports a fsbfreq of 215, forcing it down to 100 has no effect on this, it reports 215 in the bdmesg.
However, unlike my builtin fw, this works much better with pervious bootloaders as well so there is a difference.
For now I'll stick with 5.0.9 since its seems to be the latest release that behaves like it should on my "strange" phenom II.