On batteries...
Today, you can't count as solidly on the quality of Delco batteries as you once could because Delco is just a brand nowadays. GM spun its battery manufacturing business off when it spun off "Delphi". Of course, just about everyone knows that Delphi has had nothing but fiscal trauma since the spin-off. As a result, GM sources its batteries from Delphi sometimes and lots of other places too. When GM made batteries, you could count on a Delco. Now that it doesn't, I'm not so sure.
As for the Optimas...they are, indeed AGM type batteries with the "plates" in a sprial. The Optima was "invented" by another company than then sold to Johnson Controls. It is also, nowadays, heavily marketed. While the Optima design is more modern than traditional lead-acid wet cell batteries, there are more advanced choices. Also, many people are smitten with the visual effect of the spiral cells but from an engineering standpoint, the design is dumb because when you put round things in a square or rectangular "box" there's a lot of wasted space.
In fact, the Delco AGM batteries used in 02-04 Z06es are pretty good choices.
Based on subjective testing done over a long period of Delcos, Costco batteries, Optimas and Odyssey Batteries, I think, the best battery for a high-performance vehicle, regardless of price, is the Odyssey.
Yep. Odysseys are expensive but their design, performance and durability makes them worth the price, especially if you'd like a battery to last more than three years.
I'm working on a CAC product review of the Odyssey battery using a C4 and two other vehicles, but it's not ready yet. This review will also include how to install an Odyssey in a C4 which to most people is an impossibility because of the Odyssey's dual terminal design.
In the meantime, for more information on Odyssey batteries, see:
http://www.odysseyfactory.com/