What does $2000 more get me with the RX1 that I don't get with the EPL5?

The answer is it may.  Billboards are produced at 25 dpi final size. So a full frame camera gives you more native resolution to reach 25 dpi at final size.  This isn't a great example , however, because billboards can be produced at even lower resolution based on their viewing distance. Higher resolution at final size is much more critical as viewing distance decreases.