There's no question it was a 2 stage plan with incremental capex. But the 2nd stage was never a full duplicate line. Analysts specifically asked early on about a 2nd line/duplicate line and while the replies were vague it was clear that was not the plan.
The paint shop was explicitly announced as a 10k/week installation. You can search the original Eisenmann press release. The gigantic Schuler press was also spec'd for 10k/week, or at least to be easily expanded to 10k. The body shop was floor-planned for 10k, though welding lines are modular so they obviously didn't buy all the robots on day one. The seat factory was laid out for 10k, though once again they didn't buy all the equipment in stage one. General assembly, well..., who knows what the plan was there.
They're installing a new final assembly line (GA5) and new welding robots. Even that involves moving a bunch of stuff to other locations. I've heard zero about new stamping presses or paint shop. They specifically said building Y in Fremont would save a lot of capex, which wouldn't be true if they were putting in new stamping, paint, etc. Which they don't have room for, anyway.
That's why I say 3+Y will be 10k/week. We'll find out in January, I suppose.