One thing ECO mode does is let the coolant temp get down to 110deg F before firing up the ICE when you're calling for cabin heat. So it helps hold off ICE start-up...
Congrats and welcome to the club. Not only you lowered your gasoline consumption, you also lowered your overall emission from both fuels (electricity and gasoline). Not all plugins are designed to achieve both. You should be emitting about 200 g/mi down from 222 g/mi for a regular Prius. When you have more miles, come share your EV Driving Ratio here. We can easily calculate your efficiency in both the electric and gas miles (besides the ratio given).
...congrats on the PiP, when you say rebate forms I assume you know about the PA state rebate incentive.