By not having a chance to setup, i mean that one turn you use to nasty plot yourself can be enough to cripple you or get you killed, since the second you activate z-conversion the opponent throws everything it has at you. Mind you, even before conversion porygon-z has enough speed to be able to revenge kill some threats, especially if they are weak to one of it's coverage options (and they are pretty good), and you take advantage of the surprise factor of not converting but doing some damage instead. Nasty plot just basically gives them a turn to switch in something stronger, you could argue for agility instead since after agility plus Z-Conversion you'll not be out sped by anything (not even scarfed pheromosas) and you can abort after using agility.
Recover is not protection against toxic status, because toxic damage increases every turn a pokemon stays in the field and you don't switch out porygon z to avoid loosing the boosts (by turn 4 you'll be loosing 25% of your HP in one go, in turn 5 the accumulated damage from toxic alone is 94% of your total hp), so preventing that poison status is essential; you can't switch out and switch in again to reset the counter. And if your porygon z is paralysed before using z conversion to electric, that paralysis stays since your only immune afterwards (and for ghost types, your never immune). I like your analysis.