When the park opened, AK actually opened as early as 7am (that got bumped to 8am eventually -- now 9am). That was a perfect time because the animals were really active. The park would close at 7pm too, so the last ride of the day was also something special sometimes.
That's not the case now. By the park's 9am open the animals are already baked. So I'd suggest the 8am early entry, though, to be honest, it's not as if the animals are moribund even during the early afternoon heat peak. The different isn't as significant as it once was. Go on an EE day so you could make the most of your time there (being done an hour early on July will be a huge advantage in a hot park like AK).