Do you mean to just change the tuning a bit or switching between a daily and SPL tune?
I'm actually in the middle of building one right now. I have a bet with a friend that it won't be more than .5 of a difference. I'll let you know.
I don't see many of them done, but that guy on here with the BMW and pair HDC4's has a port insert. He never did let us in on the gain between the two though.