On many threequarter and half armours of the 16th/17th century you see the breastplate beeing attached over the gorget with visible leather straps at the shoulders.

Are there any benefits/disadvantages (production costs, comfort etc.) towards a design where the breastplate is worn below the gorget?

Let's see... much less comfortable and offers less protection unless the gorget is fitted and fixed to the breastplate in which case it needs more work. Making a harness with the gorget over the breastplate is just irrational.

With the gorget, often the attachment point for arm harness and whatever shoulder harness is worn, on top of the breast plate it is free to move around with the gyrations of the body, moving off center of that neck and taking the elements attached to out of position. When under the breast and back plates the downward pull of gravity produced by the straps of the B&B plates riding over the base plates of the gorget will keep in in place and centered.