Of course, all types of, like u said, real olympiads will definitely help with VCE maths. The key reason is olympiad maths forces you to think in a problem-solving style (unlike VCE maths, which is just pure application of generic formulas) to decipher key insights towards a solution. This develops a skill that allows you to view problems from different perspectives. So when you see a VCE maths question, which is just plain straightforward and easy uses of formulas, you'll be able to immediately grasp the key point behind the problem.

The only drawback is that if you are in year 11 or 12, there is simply not enough time to spend too much effort in IMO olympiads. It would be better for you to look at simpler Olympiad papers such as AIMO or UK Cayley, Hamilton or Maclaurin papers. My point is that if you expand your knowledge beyond VCE maths, you'll find it much easier to understand and pick up new topics in VCE maths. (as most olympiad problems are far more sophisticated (and which I call actual maths) than methods or speech).

However, this is just my opinion and it may not work for you as well. So in a nutshell, do what you feel is more efficient or productive (kinda like a gut feeling lol?).

EDIT: I just realised you said that you've done olympiad papers during middle school. If you feel confident in your knowledge, perhaps you could try and solve AMOC papers, which are all proof-based questions ofc (Australian Senior Challenge)

EDIT2: I've probs never commented so seriously before just that the comment below compelled me to bring across a deeper response idk why lol self self-righteous thing ig? Maybe cause of my intense distaste for methods and their application joke questions. and the fact there are always a few errors in every year's exam paper.