Well just think how many times Checo being on the podium was influenced by qualifying outside of the top ten and having a tyre strategy advantage.
He won't be able to do that with a McLaren.
He would happily qualify in 15th if it meant his first stint was longer/stronger. I guess you could say driving slower made Perez faster overall.
He'll need to push the limits from now on whenever he can and will not have the same cushion of "only" driving a Sauber when things don't go well.
Not saying he can't do it, but I think it might be tough at first.