Placing a full mountain pack in another mountain pack results in 2 free rows of inventory. This can be done for as many packs as you can stuff in.
After going about 10 deep with full packs, server performance started suffering and inventory interface stopped updating. Hive updates still worked fine, dragging an item out of the inventory onto the ground would be saved to hive immediately even with no updates to the UI.
Meeting up with another player with a stacked pack setup resulted in heavy framerate loss for the other player. Bandwidth when viewing the two of us on the player list had us at around 6000 each.
Unpacking all of the backpacks into a loot pile eventually had bandwidth return to normal levels and server performance went back to normal.