LogFAQs > #979510762

I feel like I've played games where clearing the toughest optional content powered up the final boss. You usually don't get to experience that until a second fight, though, since you typically have to actually beat the final boss once to unlock all the optional content.

For something more in the spirit of your question, my first thought was actually Undertale, although I think it's kind of a stretch to call that an RPG. Getting the "true" ending requires doing a bunch of optional side content and, importantly, not grinding, while going all in on grinding unlocks an entire alternate route with brutally hard bosses. So no matter which way you go, the final boss is still gonna be an appropriately difficult confrontation.