Physical Porygon-Z?
+Download can now trump the enemies who use more SpD than Def.
+Choice Band, +1, Adamant Last Resort definitely hits hard.
+It destroys most things that do not resist it or most things that are not dedicated walls.
+It's unexpected, and did I already say it? It hit hards.
-It's walled by Ghost-types.
-It's vulnerable to Rocky Helmet.
-Doesn't get past Steel- and Rock-types easily.
-Doesn't make use of the huge Special Attack base Porygon-Z proudly boasts.
-It's slow. Even when Jolly, it's quite slow, quite frail and much of a glass cannon.
-Aaaaand, it's STILL weaker than its Special couterpart.
+1 252+ Atk Choice Band Porygon-Z Last Resort vs. 252 HP / 4 Def Chesnaught: 342-403 (90 - 106%) -- 37.5% chance to OHKO
+1 252+ SpA Choice Specs Adaptability Porygon-Z Tri Attack vs. 252 HP / 252+ SpD Chesnaught: 384-452 (101 - 118.9%) -- guaranteed OHKO
+1 252+ Atk Choice Band Porygon-Z Last Resort vs. 252 HP / 112+ Def Suicune: 297-351 (73.5 - 86.8%) -- guaranteed 2HKO after Leftovers recovery
+1 252+ SpA Choice Specs Adaptability Porygon-Z Tri Attack vs. 252 HP / 0 SpD Suicune: 392-462 (97 - 114.3%) -- 81.3% chance to OHKO
+1 252+ Atk Porygon-Z Last Resort vs. 252 HP / 4 Def Umbreon: 249-294 (63.1 - 74.6%) -- guaranteed 2HKO after Leftovers recovery
+1 252+ SpA Choice Specs Adaptability Porygon-Z Tri Attack vs. 252 HP / 252+ SpD Umbreon: 266-314 (67.5 - 79.6%) -- guaranteed 2HKO after Leftovers recovery
Chesnaught can take the hit and faint with a Fighting-STAB in most cases. Suicune can definitely take the hit, use Scald, hope for a burn and ruin you. Umbreon will deal more damage with Foul Play than Porygon-Z itself can. These calcs are against relevant walls in the UU metagame, and clearly, the Specs set that uses Tri Attack is the winner here. Your gimmicky set could work, but taking in count maximal damage output (Adamant, Choice Band, positive Download boost), you're still weaker than Choice Specs, +1 Tri Attack.
And this is without Adaptability.
Enough with the imagination. Put it to practice. Last Resort fails if it's your only move. Without a Choice Band AND granted you don't always get a positive boost, your attacks will be softer. Return isn't as powerful. Giga Impact is too risky. In short, there are so many flaws to it that it is not worth the trouble. Stick to special Porygon-Z in this case.
