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So, looking at the line's stats, it's pretty interesting what they do with them.
Pory2 is simply an upgrade of Porygon, thereofore it's stats increase logically by 20 points each. Since Pory-Z is just a glitched Porygon2, it's HP and attack stay the same, while it's defenses drop to what they were when it when it was Porygon due to the glitch. It's BST barely increases at all.
There's no way to have a Porygon3 do that same thing without reaching legendary status with it's stats. And, to be honest...that's kind of what I want. It feels fitting that like Mewtwo, Porygon3 would be humanity's attempt at creating a legendary Pokémon. It would be pretty weird for a normal pokémon to evolve into a legendary but then, the Porygon line is weird. They could also just say the way it evolves is unknown.
Also it's worthy of note that Porygon-Z was once tied with Alakazam for having the highest sp att of all non-legends at 135. Volcarona also ties at 135 and the only things that surpass it now are Zen Darmanitan at 140, Chandelure at 145 and Aegislash at 150. I personally 100% feel it's deserving of that +10 boost to tie with Chandelure.