24000x6860px at 0.6 and 6 + soft shadows rendered in 25 hours. Not bad at all. Used 18gig. Unfortunately some textures were unattached when the file was opened on another machine with all objects and textures similar, and I still wonder why this happens occasionally. I had to re-render parts, but with such fast rendering it's not bothering me. Now just hope colors match a bit, as I didn't use GI cache.
Anyway, it looks pretty awesome, if I may say so myself. I could of course have rendered at 0.8 and 16 or so, but it's 'just' a backdrop anyway.
I can't wait to see it on the museumwall. Some minor postwork to be done. Here's a few details. I'll add some more later, if there's interest.