If Mbappe's transfer to Real Madrid takes place, a football player who will play successfully for many years as Real Madrid Benzema will be brought to the team. I believe Mbappe will outperform Benzema. What he's done for PSG is obvious and he's a perfect player statistically.
What are the bases of this your statement? You have any idea what Benzema have done for Real Madrid? Mbappe is probably a better player, but that doesn't mean he will have a better impact.
There are those that will ague that even Ronaldo did not do more for Madrid than Benzema, and if you look at the trophies they both won, Benzema has more trophies for Madrid than Ronaldo. Mbappe might score goals but lets see how Mbappe's time at Madrid will look like if he eventually signs.