I get lots of images printed for exhibition stands, sometimes as large as 3m x 4m in size. My standard procedure is to supply the printer with artwork 25% of the finished size at 300dpi, the printers do whatever it is they do? to make crystal clear prints from the file. It might be worth while running a test render to see how much this reduces the render time. If it works in your favor act all innocent with the client and explain to them the whys and wherefores of the situation and that you just assumed it is what they meant!

You might just check with the printers first though.

Good luck!