@tok 

In respect to powerline backhaul, P9 behaves just like powerline Ethernet adapters. You can Google for info on powerline Ethernet adapters, and what you'll find will apply to P9. You can start with this: https://kb.netgear.com/20233/Product-FAQ-Powerline-Adapters

Is it possible to measure the throughput of powerline backhaul?

This is how you can do that:

1. Confirm that your Satellite Deco uses powerline backhaul. See Deco P9 (3-pack) - How do I know if the Powerline Backhaul is working?

2. Connect wired device (such as desktop, laptop) to Satellite Deco Ethernet port. 

3. Run speedtest on that wired device multiple times to get better picture of throughput fluctuations. You'll be testing speed of powerline backhaul.