The rules state that when used by a non-psyker a Force weapon counts as a Best Craftsmanship mono primitive variant of the weapon. Does this mean that the Psyker gets a +10 to-hit from the Best Craftsmanship aspect of the weapon?

It does specify "when used by a non-Psyker"...So RAW, no.

Whether you deem it to be appropriate as a GM, however, is a different conversation entirely.

I always read it as, force weapons are by definition best craftsmen ship. And count merely as a best craftsmenship sword to a non-psyker. That's how I've been ruling it. Force swords are rare enough without having to get a better one for the +10 when it comes to acquisitions, and if we rule it that there aren't any best craftsmenship versions of them. Then why? I can't think of any other standardish weapon that you can't slap a +10 to hit on in some fashion.

But yeah by RAW it doesn't say that. I just took it for granted, because it makes more sense to me.