By "better" I mean higher quality. Whether that's the materials themselves and the fit and finish, etc.
"Is every custom cue better than every production cue?" No, not necessarily. I don't think I even implied that.
I think you're misunderstanding my point(s). Maybe this is partially my fault for not using a more precise word than "better."
But let's use that Mustang as an example. But let's say instead that it's a 2020 Mustang Shelby GT 500. And let's say we have another car, a Ferrari 458. Now there are certain things that the Mustang might be "better" at, such as raw acceleration. But the tolerances, fit and finish, design, etc. are of a much higher quality than the Mustang. Why? Because Ford decided to build the Shelby partly from parts made for lesser production cars. Whereas Ferrari designed every part, the engine, even the seats for just the 458 out of the best (light and strong) materials they could find.
In essence, the Ferrari is a custom car. The Mustang is a customized production car.