iOS 7 has the method:
- (void)showAnnotations:(NSArray *)annotations animated:(BOOL)animated
Give it an array of placemarks and it will change the region for you:
[self.mapView showAnnotations:placemarks animated:YES];
You can get the placemarks from the map items:
NSMutableArray *placemarks = [NSMutableArray array];
for (MKMapItem *mapItem in mapItems) {
[placemarks addObject:mapItem.placemark];
}