Same card here and OverDrive is working properly but i had to accept the terms in global settings first.
Stem uses URL shortcuts (like steam://rungameid/ID) so should be pretty easy for AMD identify them from other programs.
IIRC 8670m is GCN, not VLIW.
Installed and working fine on my notebook but the switchable graphics configuration is pretty hidden now and still using the old CCC, also, not...
Even with SSD the switchable graphics list takes a while to load here (10~15 seconds) so i don't think it's so fast or optimized. While talking...
I didn't seen a single laptop gpu with VSR enabled so far, and i have a 8850m (renamed as R9 m270x)