I went and found the write-up in Car & Driver - their quote (note quite verbatim perhaps but absolutely correct as far as syntax and message) - "The big weight savings is the 136 lb chassis - 1/3 less than the heavier but stonger steel chassis."
Like I said earlier, I'm not sure that 70 lbs saved (I know I said 40 earlier) is worth it if it means a more flexible chassis - not only from a control standpoint, but longevity reasons.
I would suspect that for those of us who would actually buy one of these cars and put it to the use intended, that we wouldn't deal much with water, so eliminate the obvious advantage of no rust that an aluminum chassis has.
You are left with a chassis that is more flexible, and aluminum fatigues quicker than steel does - If both frames flexed the same amount, the aluminum would fatigue quicker. But in this case, according to C&D, the steel chassis is stiffer also, so I stand by my statement.
We've heard for years that the vettes were always a little too heavy, and I think Chevrolet had a goal to get the weight to the parameter people have been bandying about for all this time. I'd rather have the additional 70 lbs in a steel frame, or apply another strategic 30 to the aluminum one with proper supports/mounts for SCCA rollbar mounting (thanks Mike for the heads up on that - hadn't heard about the issue) to make it as stiff as the production chassis (without roolbar installed).
In this case I will use the analogy of cars and women (male chauvinist pig that I am) and state that in both, losing TOO much weight can be a turn off...I like to be comfortable with the way they both feel.