All of this was mentioned on the "Road to PS5" stream. Sony tested 100 of the most-played PS4 games and most of them ran straight out of the box with boost mode and should be ready for launch. PS5 is too fast for certain titles. Boost mode happens naturally as a result of playing on the PS5 but this boost cannot be handled by certain games due to their code which is why they need to be tested individually but results have been "excellent".
Rest of the PS4 games will run through legacy mode.
Sidenote to all this -- But I have a feeling this is what the general shape of the PS5 is going to be.