Hey Cameron-
I took a Spanish course last term, but I'm not sure if it was advanced as yours. I think that photoreading Spanish more times than usual will really help because of the foreign language aspect.
Remember that 2-25 min sessions are probably not enough, thus the reason for your feeling of some lack of comprehension.
Try to get through the book 5-6 times (a lot since it is Spanish) and then if you need to rapid readd more slowly to make it gel, try that as well.
I know the feeling of frustration when you go to a class and everyone else has better comprehension because you photoread and they regular read. It used to happen to me, because I wouldn't activate completely - wanting to spend hardly any time on the reading. Just FORCE yourself to do enough activation layers and understand it well enough before going to class. Maybe make your activation passes shorter if they feel long? (and if it is a smaller book).
Also - are you mind mapping it? Mind map ANYTHING, even if you only have a vague idea of what you read. I cannot tell you how much this has helped me. It forces you to think about what you read and it really is important in the activation process.
If you activate properly and have a good mind map for class, you will be FAR ahead of the kids who are flipping through the book and are frantically trying to remember what they highlighted (although I don't know if highlighting is such a prolific phenemon in your classes - but kids can't get enough of it here, and I am so glad not to have to bother with it). Also remember that activation is about grasping of ideas - focus on that before you get bogged down with minute details like "Juan tiene un zapato que es rojo, tambien Juan tiene una camiseta que es blanco." (sorry had to throw that in there). But my point is - plot at first, then details.
Activation comes in layers - SOO key.
As far as your purpose, I see no problem really. It's very similar to my goals. Most of my course reading I photoread with the purpose, "I desire the information in this book/article for my purpose of understanding it well for class discussions, test and papers so that I can do well in the class" and if you want to state WHY you want to do well in the class say that as well, but it's implied for me.
Finally - I would really suggest you try PR-ing with non-school books. I had a mini "break-through" over spring break because I photoread and activated a few books which were interesting to me, NON-fiction, relatively shorts and no PRESSURE. I ended up "playing" with them and started to understand how easy the system is. The key is to practice PR-ing all the time. E-mails, these posts, the newspaper, class assmts, etc.
Have fun! I love Spanish and hope you enjoy the class for what it's worth.