I'm interested to know what you think of Frontiers in this context. Along with PeerJ I would regard Frontiers as the poster child for "large scale implementation of new systems", and judging by this blog post http://blog.frontiersin.org/2015/10/13/frontiers-financial-commitment-to-open-access-publishing/ a significant chunk of their APCs goes into the challenges of scaling beyond 1000 papers/year (Frontiers published >10000 articles in 2014, I think).