Scale length definitely impacts string tension and sound. The only way you'd really know however is if you had another 650 guitar made as close to the 640 as possible, but even then you can't be sure if the differences are due to the scale length alone. I remember having a guitar made for me that was 640 scale. When I went to pick up the guitar, the luthier happened to have another guitar that he had made that was 660 scale that I got to play and instantly heard an enormous difference in the power and deepness of the bass especially.
It's important to take all the other factors into consideration since you're having the guitar made. Consider nut width, string spacing, neck thickness, neck shape, string action... In my opinion these factors have a more noticeable impact on playability than 650 vs 640. I remember going to one dealer and he let me play two Kenny Hill Estudio guitars... one was 640 and the other was 650. He made me guess which one was 640 based on playability and I ended up guessing wrong.
One other huge factor that affects playability is the top compliance. If a guitar has a very stiff top, then the strings will be harder to press than on another guitar, even if you have the exact same string action, string tension etc. The thing with this factor is its really in the hands of the luthier.
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