I hope folks are open to this. The golf folks are ahead of us in the subject of face angle vs swing path. They have been using Doppler radar and have data that concludes that face angle contributes to ball direction 5 times more than swing path.
This, if I'm not mistaken, should also apply in tennis.
Here is a nice report. Page 12 and 13 if you want to skip to it.
http://www.trackman.dk/download/User...20Laws%20I.pdf