18K seems high. I did some tests for Universal once and we pulled 10K from some 70mm IMAX ride prints. 8K was solid, 10K was very borderline.

I'm trying to remember what resolution we rendered two face for when it went into imax.

This was using 32bit renderman, and it was constantly OOM'ing because we were really pushing it.

I seem to recall that we rendered out at 7k, even though the optical resolution was meant to be 12k.