The power boxes were located where they are because of the attach points on the seat. Power box is at every seat assembly location in order to give power (110 and USB) to all three seats.

The aisle seats already had the smallest space underneath the seat due to the seat leg and the end of the baggage bar.

As for the bins, they are estimated to allow 50% more bags to go in the overheads because they are designed to have bags turned on their side (there are placards in the back of the bin showing you how to stow).

It is also worth noting that if you are seated in 4A/C, the overhead is completely full use to the addition of a 4th liferaft. Before anyone asks, it was placed there to keep commonality for the FA's since he 900ER has two rafts up front.