I think BadBoi's point is the same as mine. To say people don't have the right to be concerned about rising gas prices because they own "luxury vehicles" is an ignorant statement. These are not Maybachs...and this isn't the LS460 or SC430 forums...
The Prius is not that light, in fact its not that much lighter than your IS:
Prius: 2932lbs
IS: 3455lbs
So its 500lbs lighter, and excuse me, the IS is a SUBCOMPACT car, two size classes below the Prius.
Its all in your head. I've seen first hand accidents with the Prius and much larger heavier cars, people walked away just fine. And no, you couldn't kick a hole in the side of a Prius. The IS "feels" heftier, but thats in the engineering, that 500lbs is in doors and sound deadening, not things that will keep you safer.
By that logic you'd be safer in an old Expedition than a modern sedan, and the evidence out there is that isn't so.