Download does not base its boost off of your Genesect's stats, but rather your opponent's stats. So if your opponent has lower Defence you'll get an Attack boost, and if it has lower Special Defence you'll get a Special Attack boost. Your Genesect's defence stats do not change Download's effect, it's based entirely on your opponent.
This means that players can adjust their sets to throw off a Pokemon's Download boost. Such strategies are common in UU where often players will use certain natures to prevent the Porygon line from getting a Special Attack boost.