While I was looking at a texture atlas using Mezz's TextureDump mod and I noticed that the size for Iron Chests textures were bigger than Minecraft's.
I was thinking that models for the different shulker boxes could be baked dynamically, similarly to either Tinkers' Construct or Chisel and Bits. It could help with the size of the texture atlas.
This won't really affect smaller modpacks but it will help larger ones.