First I'll preface this as I usually do and say wrt porting etc I am probably the slow guy in the room.
Been watching a lot of Darin Morgan talks lately and as I recall in one, he states changing the seat angle can be very advantageous depending on the shape of the bowl and how the A/F mix moves out of it, past the valve and how that venturi is built up. The particular segment also mentioned how critical not going over a 15 deg change in the angles etc. He also talked about how changing the angle can be used as a "tuning" point which of course had to be checked on the track.
I'd like to add that I find these threads fascinating and hope this and other threads continue and more begin. Good stuff.