Remember, Download raises your Attack or Sp. Attack based on your enemies Defense and Sp. Defense, not their Attack or Sp. Attack.
Smogon specifically said: "It is also interesting to note that a Download boosted Tri Attack is more powerful than an Adaptability boosted Tri Attack by 12%... On Nasty Plot sets, Adaptability is far superior, because after one or two boosts, Tri Attack will destroy even Pokemon that resist it. On sets where you have no other means of boosting your Special Attack, use Download."
They're saying that when Porygon has a moveset with no Nasty Plot, use Download, so that you get a greater boost.
In movesets with Nasty Plot, Adaptability is the best because after a boost of two stages of Sp. Attack, along with Adaptability (an extra STAB pretty much), Tri Attack will pretty much KO any Pokemon that comes in. ~