Sure the Harrop kit isn’t cheap but you can see where all the money goes in the quality of the engineering. I fitted mine myself and the ease it goes together shows just how much work Harrop put in during the design process. Then you’ve got the way it looks, as others have said already you honestly wouldn’t know it was after market.
Going back to the OP, I’m getting around 33mpg on a gentle commute and down to 29 or so if making progress.