The answer is both straightforward and not-as straightforward as you'd like, I'm feeling.

So, it's somewhat obvious where each of these link up:
Biology-->Biology
Chemistry-->Chemistry
Physics-->Physics
English Advanced-->English
Economics-->Economics
Maths Ex. 1-->Nothing - specialist maths is the closest, but also incorporates a lot of things from extension 2 in it.
Mathematics-->Mathematical Methods
The thing here is that it's not a direct relation - this is basically just as close as you're going to get. The content of each can vary fairly wildly (can't comment on economics or English, and barely remember the sciences so I'd have to re-check), with some overlap for big things, but they're also generally approached very differently in exams.
I think the scaling is kind of similar?
You can check the scaling report to find out. The study score system and your guys scores out of 100 are fairly different, but it's pretty easy to see which subjects go up and which go down.

In my personal experience, HSC often tends to be a bit harder, but it's an artificial difficulty IMO. Certainly with sciences and maths, it's not that you learn harder concepts/topics in HSC, but your exam writers are absolute dicks that give you guys insanely hard/stupid questions.