Well as you said, Elves did not spend most of their time and effort farming, but I think that
some of it *at least* was done. There's only that much food you can get from the forest.
Maybe they didn't necessarily have farms as we know them, but rather gardens, with maybe some vegetabes here and there...
And you're absolutely right about them making the most out of a small amount of food. But none the less I imagine Elrond's feast to be lavish. However, it hapened just after harvest time, so he probably has lots of food in his stores.
Plus, Elrond had to suply 4 extra hobbits with food for, what was it? - a month? That is A LOT of food!
(well, they're starved hobbits. What else would you expect of them?
)