NZQA Scholarship subjects are extremely beneficial for all NZ high school students to take, in my opinion - especially if you are doing NCEA. You’re right that the results aren’t usually out in time to include these on your application- but you can still include the fact that you’ve sat the exams, which shows that you are committed to pushing yourself academically.
I think that if you are aiming for the US then you should definitely sit some Scholarship subjects - but, you are correct that EC’s and LC’s should take priority because you will be including this on your application. Bear in mind that it’s actually possible to sit Scholarships and do well without much preparation, particularly if you choose your subjects strategically! So definitely don’t think that Scholarship has to take up heaps of your time!
Feel free to flick me an email if you want to chat more about how to use Scholarship to your advantage and the subjects you should choose for maximum results! firstname.lastname@example.org