Well, since you´re in the business as well, you probably know this already. But anyway, for a standard 70x100 cm high quality poster, a 4200x6000 uncompressed TIF in 72 dpi is more than enough. Naturally, the end result also depend on the printer and material, but  you should be able to go for 5000x5000 without problems.
If you want to be on the safe side, up the resolution with 1000 pixels on each side. It will still save you a lot of time compared to a 300 dpi render.

Rudolf Herczog
Digital Artist
www.rochr.com