It's not really as much "upper attack" as "Smash in an upward direction". It's not traditional upper attack, but screw, with pisces or horogium, does have "heavy upper smash", on it's second slopes (the parts with the "spike up" at the end). I believe this is what causes the spectacular KO's you see irregularly, while the rest are caused by the normal smash of the "heads" of the wheel. This is why choosing the correct CW is important in using screw, you want to avoid interfering with the slopes of the metal wheel at all, the two best wheels are Pisces, and Horogium, as they don't obscure that second slope. I've got most CW's, and those are hte only ones with thin enough edges/shapes to manage.
There's testing I've done in the screw thread comparing screw Pisces to screw Cancer, the latter which obscures the secondary slopes. In summary, there's a 20% improvement when using Pisces, and the increase in BIG KO's is very obvious.
This also explains why everyone overlooked screw: they were testing with capricorn
At least, that's the best I can explain my drastically better results, anyway.