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The question is does a frame with a 15mm longer chain-stay length make may difference when actually riding it and not just talking about the theory?

As you correctly surmise, you will easily be able to ride around the tiny difference. At the limit, you might not be able to, but we're nowhere near that limit when we change chainstay length from 390 to 405.

They copied it exactly, if you lined it up and measured it, it had all the same measurements. Yet his new bike rode like a shed, nothing like the original.

The external measurements matched, the material was different. That's like saying you precisely replicated 531 frame in aluminium and then found that it was a noodle (as indeed, Vitus did with their first generation aluminium frames)