Where I'm running into trouble trying to make an arugment, there really are 3 decks to consider:

Your original deck.
Your new 42 card deck with 2 extra cards.
Your original deck with 2 cards replaced.

And it is the last that is hard to measure. How do you decide how much pain you have taken by substituting the cards instead of just adding them on?
How do you model that? Most or the arguments compare the first two decks, but you really want to compare the last 2.