Odd HSD Behavior when engine is cold; preview of Plug-in Performance?

So I drove from work to a local gas station just around the block, around 1-2 minute drive. After filling up, I could tell that my engine had not fully warmed up. I then proceeded to turn onto the feeder and accelerate up the ramp to the highway to around 60-65mph.

During this acceleration I stayed in the ECO zone, though I filled it up close to all the way. I then noticed something peculiar: I was getting 75+MPG. I then noticed that the engine was noticeably quieter. It was then I saw the battery drop in power at a really fast rate (5 bars to 2 bars in 30 seconds) before the car started to act normal again. (It probably saw the battery was depleted).

Although I have noticed this behavior before when the engine is cold (it seems to rely only on the electric motor if you accelerate gently enough), I have never seen the electric motor used to such an extent to such a fast speed. I guess I also see why Toyota capped EV mode to 25mph, though clearly the car is capable of more.

This interesting event also gave me an unexpected possible preview of how well a plug-in Prius could perform. Acceleration was definitely acceptable, and maybe it could have actually gone a little faster (I didn't push it into the red zone after I noticed this behavior). I also hope then the plug-in Prius will be able to operate more like in this situation and not just during cold engine situations (due to the Li-On battery). It does also show that part of the puzzle for a plug-in version is already in our Priuses.

If someone developed 3rd-party LiON batteries for the PlugIn Prius, I can definitely see it getting around town, highway acceleration and all, with insanely good mileage.