You are a terrific devil's advocate and I greatly respect your knowledge, based on many years of reading your posts. If nothing else you've caused me to pause and think.
By all accounts the 12-24mm is pretty damn good from 18mm - that's the best part of the lens anyhow, but even so it's a stopgap, and I have a 20mm MF prime sitting unused if I sell it. In reality it would need to go - a Tokina 17-35mm f/4 or a Nikkor 28mm f/1.8 would be far more useful.
I often go to ISO 3200, and by then the D300 is pretty grainy, certainly compared to my Fuji X100, and i think the D600 will be a stop better than that.
Oh and the FX viewfinder is something I have missed since going digital.
But you're right, it's an expensive step, especially if the 24-70mm eventually gets involved as other discuss in threads below.