If I were a strategist for the Prop 8 team, I would go en banc. It’s possible the outcome would be different. Even if not, it’s likely that the dissent would be even meatier with several judges contributing - giving SCOTUS more to work with in overturning it.
As to whether the Supreme Court will choose to hear the case, I’m not as certain. Given the narrow scope of this ruling, and consequently the lack of conflicting rulings from other circuit courts, I don’t think it’s quite as good a national issue - unless the justices want to pry it open to include issues the ruling didn’t decide, as sometimes happens. It’s also uncertain what the outcome would be. I think this issue, more than most, is one on which justices would like to see their own viewpoints prevail, and that may affect whether they vote to grant certiorari. The four conservative justices may choose not to do so because they’re not sure how Kennedy will vote on a case where his ruling from years ago was cited so heavily. On the other hand, one or more of the liberal justices may gamble for exactly that reason. And it’s possible Kennedy will want a chance to correct what he may see as misuse or misunderstanding of his past ruling.