I've tutored students doing both Distance Ed and students who do Latin through their school, and I'd honestly say that Distance Ed students are relatively disadvantaged, especially in Units 3/4. The reason for this (in my opinion) is not necessarily because of the unseens (although Distance Ed doesn't provide a lot, to my ire), but because of the lack of a classroom setting for Semester 2 (Unit 4). Classroom discussion is vital for understanding the Themes/Lit Techs of The Aeneid, and without the ability to have dedicated class time used to discuss ideas and have teacher input, I believe it's incredibly hard for students to grasp a solid understanding of the text needed for high scores on the exam.
Think about it like approaching a VCE English or Literature exam - without a teacher to guide you, how confident do you feel analysing the text to the depth which is exemplified in the Examiner's reports? The Themes essay (20% of the mark) unfortunately requires an inside-out familiarity of not just the story and context of the Aeneid, but also challenges students to ruminate on the overarching implications of Virgil's poem.
This is not meant to scare people doing Distance Ed - there have been numerous people who have succeeded (achieved raw scores of 40+) through Distance Ed, it will just require concerted effort. Unseens can be somewhat "sourced" by finding (google?) the stories of Caesar, Livy, Suetonius, etc.
In terms of scores, like other posters have mentioned, Latin is quite infamous for its scaling, so I would not be worried too much about the scores, but more about how you approach the subject. Hope this helped, and good luck!