Thank you guys
@pekv2 Yes, I know for imaging but I just didn't have imaging software around and decided not to bother downloading. Anyway Windows 7 reinstall with drivers + miner, gpu-z etc took me ~30 minutes

I lost quite some time however trying to debug why can't properly downgrade to 13.1 and also searching what changed so the MHash/s speed in 13.4 got down. Failed on both fronts and just reinstalled

@Spotter Thank you, it seems like a good guide and I bookmarked it for future reference.