Hey all, OP here.

We've noticed a number of posts (on HN and elsewhere) from iOS developers concerned about iOS8 - and more specifically the new iPhone screen sizes and their impact on the asset creation/delivery/storage problem.

This is our solution from @Craftsy. It's been working great for us for months now and has lots of benefits beyond not breaking on new screen sizes/resolutions. We'd love to hear from other teams who are doing it similar/differently/better. This is definitely not perfect but as we said in the post, solves a lot of the potential problems.