I've often wondered about this one myself, and unlike the case in which he was offered to become supreme leader of Peru, now he has a complete nation at his disposal along with its resources.
Well, on one side he's still allied with Pueyrredon, on the other, he's image in the United Provinces is going to deteriorate even further. The Unitarians specially hated him, and now he's used Government resources and men to make himself head of state of another country. This could ironically mean the unitarians oust Pueyrredon and worsen the Civil war in 1818, potentially causing the directorate to fall two years early.
Another interesting fact to take into account was that now O'Higgins' party is now being represented by a foreigner, and O'Higgins has many enemies within the Independence movement and outside of it, such as the Carrera brothers amongst others.
Now, naturally, San Martin uses Chile's resources to create a more apt and larger expedition to Peru...
He otherwise ignores many issues within Chile, treating it only as a platform to invade Peru once he finishes his campaign in Chilean Soil and proclaims the Constitution of 1818.
Of course, he can also use Chile as a model in which he shows what his vision of a Free American nation looks like.
Otherwise, he makes less enemies than O'higgins since San Martin is more concerned by the Peruvian campaign and doesn't have O'higgins' autocratic streak.
I'll get back with more details later...