Where do I begin? I've studied these cars a bit and have some info.

First. Geo Storm = Isuzu Impulse, hence all Isuzu parts, no relation to other Geo's.

Second. Geo Prism = Toyota Corolla.

(The GSi models of both cars happen to look a lot alike, 1.6 DOHC, 130 HP, FWD, strut suspension, but that's only coincidence.)

The Prism GSi got a motor was unique to it. No other Toyota in the U.S. got it. Yes, it's similar to other 4A-GE stuff (I forget the exact designation for the Prism) but it makes more power via a different head and intake. The FX16 and MR2 got the lesser version. MR2 and Prism share brakes, but the FX16 has smaller brakes.

The MR2 is a good ITA car, while it's down on power from the Prism GSi ITB car, it's mid-engine configuration should be faster on the track.

The FX16 though is basically an underpowered, underbraked, two door Prism GSi with the same wheelbase, and almost identical suspension pieces. But I'm drifting a bit.

There are a few Storms and Prisms out there being raced. They must not be class killers, or we'd hear more about them, right?

Or perhaps horsepower to weight isn't the whole story?

And if you're wondering why there are 16 valve 1.6 liters in ITB, what about the 16 valve 1.5 liter Honda in ITC?? Shouldn't it be a real class killer? With some development, it ought to do really good. Oh, wait, one won the ARRC. In like it's 4th race.
(No offense the the Zen Racing gang, they build great cars and are great drivers. Want to win the ARRC? Drop them a line.)