I took the Insight up to 90 MPH for a while today and only got 25 MPG while doing it. Sport Mode + Econ On.
I do see a desire for having a rear sway bar installed, though. But it can be compensated for by driving intelligently and planning your actions in advance.
Still, it did not feel unsafe driving through a twisty portion of 77 as long as you did not abruptly try to change lanes at speed. Letting the road do the lane changes for you it was mostly a non-issue.
I am surprised however how responsive the car was at high speeds. I guess I can see why the Insight hands the Prius its rear end at highway travel.
edit: I did have the wife with me, we had to be somewhere effectively yesterday for an appointment and running late. She pointed out the engine noise. I told her ignore it, Honda's love revolutions, it's designed to do that. lol