I'd be so surprised if GDR3 brings notification system and independent volume control along with all the other new features (orientation lock, 1080p, quadcore support, new live tile layouts). I would have thought they would have kept those two features for 8.1. Unless GDR3 is 8.1. Now that would be an epic twist.
Notification system isn't going to make the cut, it's too late.
The intent of GDR1 was a few tweaks that just missed the cut for WP8. GDR2 enables Snapdragon 400 support, potentially 2GB RAM, and probably some more stuff under the hood that enables some of the Amber tweaks. A few features, too, but primarily hardware stuff. Original intent was GDR3 was Snapdragon 600/800 and higher resolution, and then WP8.1 covers the software side. It's not like new hardware needs to wait for WP8.1, anything hardware driven is enabled in GDR3.
So, keeping this in mind, there's no real reason to hold back features for WP8.1. It's not like Nokia's going to hold back on releasing the 930 because of WP8.1, they'll just release it on GDR3 and then update. Only impact keeping stuff in WP8.1 has is having a less fully featured OS between the release of GDR3 and WP8.1, which isn't something anyone wants (other than GOOG and AAPL.) As long as it won't delay GDR3, it goes in. As soon as WP8.1 got pushed back from the original intent of October this year, the scope of GDR3 increased. Independant volume controls are ready, so they're in. Notification center isn't ready, waits for WP8.1.
The orientation lock was another thing probably originally intended for 8.1 that got moved up.