Eaglejwd
Actually, I'm of no actually practical help as I haven't tried mounting New(er) tang sights to an original 1873. However, the only way a tang sight would affect ejection is if the sight interfered with cocking the rifle.
The Carrier Block is actually the "ejector" and relies on rapid rise to kick out the fired case. Anything that inhibits the Lever Travel could have an effect.
I have always questioned a Tang sight on anything other than a Hunting arm as the the Tang sight is really in the way.
Have Fun Out There