Alrighty, so I finished the fight against Izanami just maybe four or five days ago and, I still can't figure this out...  
 
I believe I was level 79 or lower, and I fought Izanami with a level 81 Satan. My question is; are those level 90 Persona's meant to be for the second playthrough? Did I miss 10 levels of Experience somehow or is there an extra 10 levels of Exp in the Second playthrough?  
 
The only optional boss I did not do was the Heaven one; but I only ever leveled up like 1-2 times in the optional dungeon-run & boss. 

You got the true ending....how could you have done something wrong? Max S link personas are for nerds like me that grind for no other reason then to complete the compendium.

No, you didn't.  You just get access to those Personas by spending more time in the dungeons leveling.

Of course you did miss out on the best item of the game by not redoing Heaven-1/2 SP cost.  But yeah, P4 isn't like P3 where getting up to level 99 is just a unlocked dungeon away.

:O... 1/2 SP Cost??? Oh darn... well I kept the save data before the fight anyways, as well as the New Game + data so I guess I could always go back and do the bossfight again without using a Soma... Oh how Naoto could desperately use a 1/2 SP Cost.  
 
So what you are saying is, to get the grandaddy of all Strength Arcana Personas, I'd have to grind out another 10 levels? I guess it would have been cool to fuse the best Personas of each arcana but damn, 10 levels?  
 
Thanks :D

Yeah unless you want the complete compendium, not a whole lot of reason to get up to level 90+ . Some of the best personas(personae?) in the game aren't even that high level to fuse. But what the hell, not like there's much else to do after you get the true ending. And an excuse to put more hours into this great game is an awesome thing, regardless of "pointlessness"

Yeah, now's a great time to experiment with fusing some cool personas that will be useful should you do a new game+. Getting the high level skills and passing them down to more affordable personas will make the goings easier.