While I tend to agree with a lot of your suggestions, there are a couple that don't quite add up for me. I'm a Mechanical Engineer that designs building services, and while I don't do factories, much of the plant and equipment used is similar.
No.6 I expect will be a very heavily serviced building, possibly even up to cleanroom requirements. I get this from the large number of footings and penetrations on the roof that appear to be in preparation for a large number of air handling units (RHS and along the centre). This is similar to the finished power train building but looks to be more intense. It would also make sense to locate such a heavily serviced building next to the central energy plant behind it. What could require such heavy servicing requirements? My guess would be cell manufacture or similar like on the earlier Giga4 Berlin plans that showed a Cell building. (Although it is a bit remote from the battery pack building ont he other side).
No.8 Isn't air handling and wouldn't be specifically for the paint shop. The units on the roof of this building are cooling towers, used for heat rejection from chillers in that building and for rejection of heat from remote equipment. Pipework from this central energy plant building carry chilled water for air conditioning, condenser water for remote heat rejection/cooling and heating hot water. There could also be some compressed air and possibly standby power generators in this building. Air handling is generally located adjacent to the area served due to the size of ductwork and inefficency of reticulating distance compared with pipework.
The other thing to consider is that much of the floor area of these buildings is two or three stories while the Model 3 plant was mostly single storey. This allows plenty of extra space for other activities such as plastics molding, seat manufacture, logistics, offices etc. One of the earlier plans for Giga 4 showed a lot of these activities occuring on the second floor above the main assembly areas.
