Alexander,
Why not? Current e.g. xfce4-mixer source version IS the same in both jessie and stretch (
https://tracker.debian.org/pkg/xfce4-mixer). The real problem here is that several XFCE components in jessie (whose source version has not changed in stretch yet) conflict with xfce4-panel 4.12, so they were binNMU'ed in stretch. In order to get e.g. xfce4-panel properly backported in jessie, I need to rebuild them against backported libs (with my proposed suffix "+b1~bpo8+1").
Anyway, I'm trying another approach: a minimal backport with only xfwm4 and its dependencies. I'll try to install them in jessie and see if it fits my needs.