Sorry, Oshyan, you're right of course. I'll manage on my i7 I think (I haven't purchased it for nothing), as a 6000px wide image renders in just over 2 hours at slightly lower settings and no soft shadows. And for such a large image, viewed at a little distance in a quite dark subterrainian parking lot, I think it needn't be thát perfect.
I'll try the 1/4 resolution GI pass first. Then set the resolution on what is needed, and render out 4 crops or so.