The Airbus goes across the Atlantic because you can do that with a 120 minute ETOPS approval. The A320 family however is restricted to a 120 minute range because that's how Airbus certified the aircraft. It can't do a mainland-Hawaii flight because that requires a 180 minute certification. I suppose if Airbus ever wanted to, they could go through the expense of certifying the 320 family for a 180 minute ETOPS, but then they'd lose a lot of range because, just like the 737, they'd have to carry extra ETOPS fuel, and run the APU the whole way across the Pacific. It just doesn't seem reasonable that they'd do that.
Therefore - Virgin America is not doing 'proving runs' to Hawaii. Like an earlier poster said, it's a delivery flight. VX may be aiming for ETOPS flights, but that would only be for the Atlantic/Caribbean/South America flying, not Hawaii.
HAL