JoeJoe wrote:
Considering that these fan are only designed to work to their full capacity
with straight ducting of 2-3m according to the manufacturer (mine will be a
pretty messy 4-5m), I just thought that I could get away with using smaller
ducting. Is the only drawback the drop in performance, or is there anything
else I need to consider?

I would have thought the only other problem might be that of
condensation potentially collecting within the ducting; not that that's
not an issue with shorter runs, just that it's likely to be a bit
harder to get a reasonable and consistent fall on the ducting if it's
going to be 5m long.

Changing from round to square cross-section will inevitably clobber the
performance as well, but I expect you're aware of that.

David