I don't agree that the Insight is slower:
..........Insight Prius
0-60 time...10.7......14....sec
top speed...112.......97....mph
Also, your definition of "slightly" seems to be different than mine. The Insight w/ manual trans gets 68mpg highway, vs 45mpg for the Prius. In city driving, the Prius gets 52mpg vs 61mpg for the insight. So the Insight gets 51% better economy on the highway, 17% better in the city (with manual transmission). For the CVT-equipped Insight, the numbers are 57 city, 56 highway, which puts it at 24% better on the highway and 10% better in the city.
I'd also like to correct a misstatement made by the guy who wrote the site you referenced. The Insight is not a "serial" hybrid (I assume he meant "series," although perhaps the car is small enough to be a "cereal" hybrid). Since the electric and gasoline engines can simultaneously drive the wheels, it's got to be a parallel hybrid. A true series hybrid would have the gas engine completely separated from the drivetrain (as part of a gen-set).
The Insight's battery pack comes with an 8 year / 80,000 mile full-coverage warranty. It is, I suppose, somewhat similar to your description. The battery pack is made up of 120 1.2v NiMH batteries in series. The energy storage of each cell is higher than what you'd typically find in a NiMH D-cell, if I'm not mistaken, at 6.5Ah. (Typical NiMh flashlight batteries are in the neighborhood of 2Ah or less, if I remember right.
If you want the extra space offered by the Prius (and it's by no means "roomy"

, but would prefer even more room, zippier performance, and better fuel economy, perhaps you'd be well advised to wait for the Civic Hybrid, which is supposed to come out this fall.
Cheerio.