Mopani Driftwood/pH

I'm about to get and add 2 big pieces of Mopani to my 75 gallon tank which already has a low pH of around 6.4 and thats without CO2.

How much of a drop should I expect? Its never been presoaked so I might have to go that route even though I won't mind the tannins.

Is the pH drop going to be any different with 2 pieces then 1 piece?

If I filled a tank with nothing but wood and water whats the lowest it could drop? I figure theres got to be a limit.