I finally got to the end of disc 1, here's my opinion on the rest of the bosses so far.
In the Wutai quests, Rapps (at level 32) was ok, his Aero3 (Aeroga?) was a one-shot-kill attack but it wasn't such a big deal, I think he's fine.
I tried the pagoda immediately after, and couldn't get past Chekhov, because there was no way I could keep myself alive and outdamage her Absorb, so I leveled to 34-35 and got Regen. After that, all the rest of the bosses (even Godo) were ok.
The Red Dragon was kinda harder than the Demon Gate (at level 38-39), but neither of these bosses was particularly hard. I think you should buff Demon Gate's HP a little. After slowing him and keeping Regen on myself I barely had to cast any cure spells (I kept everyone in the back row of course).
The bosses guarding the Kjata summon were a pain (at level 41-42). It took me about 5 tries to figure out an effective strategy, and another 5 to manage to execute it without them destroying me with Thundaga/Blizzaga... Then I finally realized there was no way in hell I could beat Levant when he was alone. So I just left one of his goons alive and blasted Levant with spells till he was almost dead, then finished them off both with Limit Breaks.
The main idea was to use Odin's Gunge Lance and Magic Hammer to drain all of their MP, while keeping the party alive with Regen (or White Wind if they used Reflect on me), and dispelling at every turn so they'd waste one of theirs to recast the buffs.
Jenova-Life (at level 43) was WAY easier than the Kjata bosses. I basically just used Esuna or White Wind to dispel sleep and Regen to help keep the party alive, while DeSpelling her Reflect and draining her mana with Magic Hammer. After a while she had no mana to use any attacks, so I just hacked away till she was dead.
Moral of the story: Magic Hammer is too strong. It seems to be the solution for a lot of otherwise very difficult boss battles. Make it absorb 75 or even 50 MP instead of 100.
Also, I find myself not using Mighty Guard because of the high mana cost. You probably should revert it back to its original cost, but keep the single targeting. As it stands, I'd rather use Haste-All and Barrier-All.
EDIT: I also think you should increase gil gained, even just from bosses. I actually had to farm for gil to buy equipment/materia, which kinda defeats the concept of not overleveling.