So I moved the Debian 9 rt disk to another computer (AMD quad) and it's pretty good (seems like my other computer had some hardware issues or whatever leading to xruns).
5.3 ms latency for FM8 and various Amp Sims with the inbuilt sound is stable with no xruns.
So the Debian 9 (stretch) with the rt kernel latency performance seems to be pretty good.
The motherboard inbuilt sound is whatever, and dedicated audio devices should do a lot better unless there is some sort of problem (maybe drivers etc).
For Debian 9 I installed the rt kernel from
http://snapshot.debian.org/binary/?cat=l
linux-headers-4.9.0-3-common-rt_4.9.30-2_all.deb
linux-headers-4.9.0-3-rt-amd64_4.9.30-2_amd64.deb
linux-image-4.9.0-3-rt-amd64_4.9.30-2_amd64.deb
and installed rtirq
rtirq-init_20150216-2_all.deb