No, castings do not provide an easy to compute upper bound.
Just as with batteries, mining, and demand, casting capacity can be increased. That's how production grows. In fact, the casting machines take up much less space than the many robots they replace. And to think that Tesla can constantly grow production but that the leading innovator in the manufacture of casting machines cannot, that it is a company without the ability to scale and grow, is short-sighted in the extreme.
Whenever you have a company growing as rapidly as Tesla they will hit "the ceiling" temporarily in many ways. There are always snags to growing. But all of the limits to growth are transient in nature, there is no upper bound.
The other reason casting machines do not provide an easy to compute "upper bound" is because the rate at which they churn them out is not fixed. Making castings this large is a new technology that requires new knowledge and is subject to the same kind of innovation Tesla applies to all their processes. If Tesla can, on one casting machine, figure out how to make a two minute process happen in 1 minute and 30 seconds, they can apply that knowledge to all their casting machines and make 33.33% more cars per year with the same number of machines.
The "upper bound" is not fixed and nothing about this is easy to compute.