A little update on the issue:
Just confirmed that the cache file indeed does work.
However I found out as well that the cloud quality is to blame if you are doing crop region. Apparently, the
higher cloud quality the
more likely it is for the bucket seams to be visible.
I'll experiment with these values yet.
Strange, I know but results speak for themselves. I cranked cloud quality to 1 just like Oshyan suggests in rendering tips thread and it worked just fine with clouds still looking very nice and bearable render time. No seams visible. So far so good. Gonna post results NLT Thursday.