I know nothing about the N20 or the f30 but let me attempt to provide some insights from logic.
1.The n20 in '20s and 28s are mostly identical because BMW wants to reduce costs.
2.It makes sense to differ a few components to make it uneconomical for people to buy '20s and remake them as legit '28s. But not too much (as it costs more), just up to the price difference in the models.
3. Binning engines is stupid; if the differences are so large, the engines would fall apart!!!
4. You can make a '20 as fast as a 28 without changing the hardware differences, by a cheap tune. Although it is not exactly the same, the performance is.