Hello!
I'm doing methods 3/4 this year, albeit using the Jacaranda textbook and not Cambridge (though I also did some questions from Cambridge in my spare time). This is purely my experience but to answer your questions:
How early should I complete the entire Cambridge Math Methods 3/4 book (excluding the review chapters)?
While there is no set time to complete it by, you generally would want to be doing it at a quicker pace than the one the school set. Many of the students in my cohort who are ranked towards the top end finished the textbook around term 3. For me, I finished the textbook during the holidays between term 2 and 3.
How beneficial is it even to finish it early?
While this will depend on the individual, generally it's beneficial as
1) If you burn out some time during the year and really need to take a break, you won't be left behind
2) If it takes you more time to fully learn a concept, starting early will give you a more solid foundation
3) You will be able to identify what areas you struggle with early on, so when your teacher goes through the concept during class, you will know exactly what areas you need to work on.
4) Textbook questions aren't exactly up to the level of VCAA exams. If you finish the textbook early you'll have much more time to familiarise yourself with exam-style questions, which are much more important.
I have heard 50 SS scorers covering the entire syllabus before the year even starts!
Yes, this is true. I also know people who covered the 3/4 textbook before the year started but ended up with study scores in the mid 30s. Others finish the textbook much later but still end up with high 40s - 50 study scores. Do not rush through the textbook for the sake of finishing early. Make sure that you are building a solid foundation for yourself.
Also, do not try to do every question in the textbook. Generally, it's the worded problems that you'll want to focus on.
I hope this answers your questions! If you have any more, feel free to ask