Big batteries also put uneccesary strain on our "topping-up" alternators. Our alternators are not high-capacity shop-type rechargers.
Of course....there is no "Porsche size".....I mentioned that the Optima to use will require the fabrication of a base plate that gets attached to the battery....and the whole shebang then gets placed into the battery tray and uses the rear "lip" in the car to catch the baseplate. It's all in the archives including templates.
You will also find that the terminal post locations on Optima are on opposite sides than normal, and you will need to fit a longer ( 9"-14"...forget the exact size) longer ground strap. Not a biggie....mounts to the chassis nearby via a screw/bolt.
Wasn't aware that the newest Optimas don't have the small mounting holes at their base...maybe they can be drilled where they always were.
Here's a thread to get you started-->
http://forums.pelicanparts.com/porsche-911-technical-forum/583506-what-size-optima-89-carrera.html
Another one, pics, template, battery size, etc---->
http://forums.pelicanparts.com/porsche-911-technical-forum/328799-optima-power.html