Here in Australia. I currently have 8GB data and I almost turn everything ON, except app updates and reading list, and background app refresh for many apps.
I barely see any serious data leak in such a crazy manner, unless I am streaming YouTube video through app. Maybe I am lucky. Maybe I keep restriction on all the time. I am glad I have no issue OP mentioned.
Oh, Optus 4G is blazingly fast, I must say, especially when coverage is excellent. If I have the same issue OP has, then going away 1GB data would be only a matter of minutes. Now I am 11days left for my current bill month, and I have 4.58GB left.
My own experience.