Why would you accept something that, if not tampered, would be better than the alternative? When there's bad intentions or tampering, you'll have a result that is broken to some degree for sure. If everyone does their jobs as good as they can, you'll only face a chance of something being broken. You can always improve over mistakes, not so much over bad intentions.
I never checked this, but I always thought that slower CPUs are just CPUs with some 'production faults' in it. Basically, the manufacturer always tried to make 300mhz CPUs, but sometimes they didn't reach that speed so they sold them as 233mhz, 166mhz.
Now I was under the impression that most people didn't know this, so most people were buying CPUs which were just designed to be slow, but in fact they were accidentally slower.
This is exactly your car example, and there is nothing unethical about it -- either way.
You're trying to trick the consumer into thinking a human made the sign (and by extension the product as well), which was actually made my machines.
I don't see a difference. When I buy a product, there is no disclosure of the manufacturing process.
I could only see a good lawyer losing this case if OP signed a contract that explicitly stated the process in which his work must be produced.