OP, I'm in a similar situation, moving from a 4 to a Note. I've completely nailed down all the possible data hogs
but I've just come to accept it uses way more data than my old iPhone. A couple of times I've had to add on an extra 500mb to my O2 contract. Thankfully, said contract expires next month , at which point I'll be going to the excellent Three monthly rolling contract with unlimited data. Can't wait, I've used Three data with my iPad, far better and faster than my experience with O2.
I think the main culprit is actually from browsing in desktop mode, whereas previously a lot of sites were optimised for iPhones and low data.