The speed of their metabolism is related to water temperature: by keeping the water that cold, the Long Beach Aquarium extended the life of the bimac in question. This does not in any way mean that 59 degrees is an ideal temperature--far from it, you could argue that the Aquarium unnaturally extended the animal's life.
If we want to think about the "ideal" temperature, the starting point of our search should be the temperature range of the animal's natural habitat. Lets see if Google can help.
NODC Coastal Water Temperature Guide
As you can see from this, the water temperatures off southern California seem to vary from a winter low in the upper 50s to summer highs in the upper 60s. With the internet, the world is at our fingertips!
edit: I think the point here is that under no realistic circumstance should you expect a pet octopus to live for two years. Very generally speaking, these animals have a simple life history: live for a year, lay eggs, die. Most successful bimacs have been sold by a breeder at 4-6 months old, meaning you can only reasonably expect 6-8 months with one. From a practical standpoint it really is less, assuming multiple weeks of senescence and the fact that most hobbyists can't match the cool water temps in the summer. I try to chime in whenever I hear someone bandying "a year and a half" or "two years" around here or on Reef Central, because this is just plain unrealistic. It is unfortunate but true and too many people pursue this hobby with unrealistic expectations. People need to be honest with themselves and decide if it's a deal breaker that their pet will only live a few months.
Dan