Don't know if this will help you, I found the info on a bike forum.
Changing the clearance will effect the timing slightly, tighter will advance, looser will retard. It will also effect the lift, tighter =more, looser = less. It may effect duration the same way, tighter= longer, looser = shorter.
Will the average rider on the average bike notice these minor changes, I doubt it. On a highly tuned racer with a highly tuned rider then maybe.
Steve