The 20mp 4/3 sensors have noticeable advantages in dynamic range and high-ISO noise compared to the 16mp sensors. It's not just more pixels. A new ~23mp sensor with further performance improvements should show up this year in the Panasonic GH6 and an unnamed camera from the company formerly known as Olympus, though it's not clear if or when it will make it to more pocket-friendly models.
Perhaps, but I have a Panasonic G9 (20mp) and a EM1 Mk1 (16mp from 2013) the differences, while measurable to some degree in benchmarks, in day to use are really not that noticeable at all. The m4/3 industry relies effectively just on Sony now for sensors as Olympus/Panasonic do not make them, and there is not that much commercial incentive for Sony to invest more - this is why we only get a sensor feature bump when the Sony sensor catalog finally gets a new installment in 17x13mm.
I noticed a big difference going from a Panasonic G7 to a GX9. It gave me a stop more usable ISO and a lot more ability to bring up the shadows when editing.
