There could be room for a 2nd package.
But I didn't know about the E5 V2. Maybe you're right and Apple is using that. E5-V2 has 4 DDR3 memory channels, Apple would only have to put two DIMMs on a memory channel to have 8 DIMM slots. They have two DIMMs on a memory channel in the current MacPro.
But I suppose it's more likely Apple is going to just stick with 4 DIMM slots, given that memory per DIMM availability probably has at least doubled since Apple selected 8 DIMM slots for the last Mac Pro (2010? 2008?).