User blog comment:Miskos3/Fairy Tail 521: The Strongest Wizard, Review/@comment-5894890-20170212010455/@comment-1153699-20170214200250

I see your point but I'm not sure that's how it goes in FT.

Max actually gave Pre-SO Natsu a fight while SO Natsu humiliated him like he always would.

The only difference between the two Natsus is Second Origin and there's a clear difference in performance between them. And what Second Origin is, is basically a MP boost.

It was also portrayed that Natsu and Gajeel have both gotten stronger in comparison to their Tenrou selves. Natsu only had his SO unlocked while Gajeel was training as hard as he could for 3 months. http://mangalife.org/read-online/Fairy-Tail-chapter-294-page-30.html

I doubt having more "Mana" and the benefit of casting stronger spells with less effort are the only buffs you get when you're having your MP increased.