True. It is pretty close to a 'zero sum game'.

The DME will retard timing a bit to prevent the pre-detonation that lower octane gives you...so power and performance will be down, mileage suffers a bit.

Does the 20 cents a gallon (so 5% here in CA) offset the 5% worse gas mileage?

Even if the above is not arithmetically perfect, you get the idea.

Over 100,000 miles or regualr, will engine life suffer? Who knows? Why bother?

I actually run 95 octane in the M5 and 100 in the 996TT.