ECs, and even the essays, only apply to borderline students with SB. If your main aim is to be a Gaucho you should probably place everything else on the back-burner and work primarily on UC-GPA and completing your pre-reqs as SB is one of the more stringent campuses when it comes to completion of prerequisites.
Source: http://admission.universityofcalifornia.edu/counselors/files/transfer-admission-matrix.pdf (Page 9)