September would be cutting it close in my opinion for letter that will help your application from an interview standpoint at other institutions. Why? Because even if you do a 2 week elective, you will not ask for this letter until the end of the rotation. Your application will be submited prior to mid/late September. Sure SF will distribute it, but it will not arrive until October..... Programs will have already printed out the applications prior to this.
Away rotation can help you obtain interviews - it could help you obtain an interview at Bascom, but you better be ready to shine on day 1. You have a limited time to make an impression and opinions will be formed very quickly. If you are the type of person that all the residents/attendings like on rotations (in all of your rotations) at your home institution, away rotations can surely help. In general away electives should be done after you have done a rotation at your home institution so you are ready to go and know something about ophthalmology.
In general:
1. If your heart is set on a particular program, I feel away rotations can help you get there if you have the right skill set/personality.
2. Away rotations can help getting a stronger letter of recommendation if your home institution is weak in ophthalmology.
3. If your home institution is strong in ophthalmology and you do not have your heart set on a particular location, away electives are not needed.