Um the stock internals are plenty strong enough to handle the modest boost of a procharger at an average of 350-375rwhp. Now if yoru talking about boosting past 400rwhp then yes it's a good idea to redo the engine internals on the Z.
As for the weight 3100 isn't to bad though it is a bit heavy I can't seem to find the weight of the WRX on the website but I was just imagining being a sedan it's goign to weigh in more but I could be wrong about that.
As for the handling yes all things being equal evo should handle better but the truth is being able to drift can really give a huge advantage in saloom or twisties. Drifting in a AWD car isn't really possible.
As for the WRX chasis being superior why do you say that? I'm quite curious.]