I think racket face angle, swing path angle and swing speed all contribute to spin. I think a closed face on the same swing path angle and at the same swing speed will produce slightly more topspin. And, yes racket face angle also impacts departure trajectory angle. TW has a study on serve where that have "proven" that hitting the top 1/2 of the ball produces more spin. A closed face on the topspin FH is in effect hitting the top 1/2 of the ball and you encounter a smidge less resistance at impact and maintain a higher racket speed thru contact resulting in more RPMs on the way out. Practically, I don't consciously muck with racket face angle and just use my hands in conjunction with my swing path to get the up/down trajectory that I want. I have accepted that a slightly closed face is good and use this on topspin shots but again, I never think I am closing the racket 2 degrees more than normal. The racket face angle just sort of happens with practice.