I know a lot are waiting GDR3, I for that matter am waiting for black, as i had GDR3 for months now (and a huge nomber of people on this forum do), and TBH the only thing i find important in this update is battery! the battery is amazing now.
You can always upgrade to GDR3 (without the "black" firmware update) by using a developer account and App Studio. The build available to developers is the same build as the one released, so minus firmware updates for the phone, you can upgrade your 920 to GDR3.
Not that much of an improvement in terms of what? 1520 has killer specs, the size is definitely an issue though, especially for people with smaller/average hands.