Refining may have made improvements, but I don't believe it has very much impact. Just look what happens when single refinery goes offline or has to manufacture designer fuels for Kalifornia or heating oil for the north.
Why has diesel fuel jumped ahead a gasoline prices in the last couple of weeks?
That's one of those "problems" that don't need fixin' from a certain point ($) of view.
Diesel has jumped on expectations of demand for heating oil (which is somehow related to diesel, don't ask me how). As for gasoline, we are coming out of the peak gas usage season (the summer), so we'd expect gas prices to fall.
The problem isn't on the demand side, it's that traders are worried about the supply side, so they buy up heating oil contracts now so they'll have the heating oil when people need it in december. ANWR would have directly address supply on two fronts: by providing placing a large reserve on the market, and by creating a stable flow of oil not subject to the political insanity of most oil producing countries.