I would think that more stops closer together would save more money then adding sea days where the enginges are using fuel at a higher rate.
As I really enjoy the longer cruises, I think adding a day to T/A's and roundtrips to Hawaii and going slower would save on fuel.
As far as food, the ship wastes a a huge amount of food in the buffets where they don't know how many people are going to eat there. Close the main dining room for breakfast or lunch and have everyone eat in the buffet, or close the buffet for one of the meals and have everyone dine in the main dineing room. Lobsters- serve two tails with the dinner and do not allow diners to order additional lobster meals, they can order anything else on the menu but are limited to just the two lobster tails. Thus saving money and preserving one of the great perks of cruising.