Knowing and understanding the subject matter is likely to be pretty useful if the test matters at all. This is why you study and understand the entire course, which is the point of the course. However, being told explicitly which portions you will be tested on goes against this goal.

which of the 5+ page proofs of theorems from the course wouldn't need to be known for our exam - it wouldn't have been very good use of my time to fully internalise all of them

What? If you're doing mathematics correctly, you are ably to derive the proofs on the fly. You don't have to memorize "five pages of proofs". You understand the concept, you understand the mechanics, and you derive the proof as necessary.

I think you just have an idealised picture of how tests work.

I'll refrain from saying what I think about your "picture of how tests work".