Internally, the Z8 will lose many of its expansion slots if you're using three dual-slot GPUs, but there is an empty PCIe x8 slot at the top of the motherboard.
The shrouds also ensure that the second processor ingests separate air from the first CPU, because HP suggested that the second processor ingesting the first processor's hot air has a negative impact on the performance and longevity of the second processor.
Two of the PCIe x8 slots have no access to the back of the machine and are intended for use for solid state drives and other internal expansion cards.
For internal storage expansion, the Z8 has four.5/2.5 bays, and it has two.25 external bays (and a slimline bay used for the optical drive.).