Well, Reborn is meant to be hard and I`m not sure if Hyper Training is this necessary. Maybe few bottle caps here and there, but really just a few,for the sake of convenience. Of course, the closer to the end, the more IVs and EVs matter; I tried to ignore stats and just play with mons I like, it is possible to go through the game like this, but when I saw the difference in how my Gardevoir performs (in the story, not online) with and without proper EV training... Well, perfect IVs and EV-trained mons make this game even way too easy, but on the other hand you just can`t completely ignore stats. Even if E4 and Champion will be almost competetive-like, for me, softresetting few times to get just decent stats and keeping in mind you need to avoid overtraining, say, special attacker in defense, is enough. Especially when we got access to ability capsules and nature changer in 7th Street. Strategies matter more than brute force and outspeeding enemy here.
By the way, while there is some fun in doing eugenics programs for IVs or moves, composing team to cover weaknesses, etc., in the same time I see how I lost the trace of the atmosphere I liked in Pokemon - you know, this is my monster, I caught and raised it, I fought leaders and went through major events with it, kind of attachment. Instead, they became soulless pawns, tools - this one is weak, so dispose of it and fill another 4 boxes with spare mons while breeding for better model.