both def and mdef are doubled by the engine, and yes it is true that enemies would need less room to have their def to go up to 510, since it is doubled initially. but, the thing is, enemies shouldn't need to almost null damage (255, the base max for the defenses halves all damage, except def ignoring things, and 510, the doubled max, would take the damage to pretty much nothing, around 1-100 for powerful attacks, since the damage done is damage = damage * (512 - def) / 512, making 10000 damage taken down to ~40 damage dealt, a really big decrease. if an enemy had that much defense, then it is likely that they wouldn't have much life, or the fight would be really unnecessarily long, especially since hp can go pretty damn high.
yes, an enemy with 400 defense (200 base) would still take plenty of damage, but if you had a good ai, great moves, and ample hp on it, then that high of defense wouldn't really be needed. the main problem with most of the vii fights, is that the enemy ai is too simple, and their moves too weak. many enemies are too low level to be doing a ton of damage, and some at very high levels would still be very easy since their ai sucks. if you balance out the game well, and make it hard from there, then mass def boosting isn't really necessary, just overkill, and making the fights much longer than needed to be.
i guess that the def doubling is good for now, and will still be needed when my first compressed ai scene.bin is done, since the enemies will still be essentially the same, but when i make my REAL difficulty hack (want this done so that i have plenty of breathing room, and so that psx version users can still enjoy my hack, since it should still fit into the original iso easily), i am hoping that i will have a way to stop the initial doubling, even though it is likely many enemies late-game will be hitting the higher stat marks, and the end game optional bosses will definitely be taking roids, so only a few enemies should need past 255 defense, if they even really need it at all. usually, if you know the enemy ai very well, and have a very nice setup, you should eventually win anyway, but my hack is going to be cruel to the users, so massive defense would just be too much .
well it is a problem with too low hp/def/mdef, I defeated red dragon without it even getting one attack on me, so you could give red dragon very high attack stats and a brilliant AI but it means nothing if it doesnt get a move in, this red dragon I defeated in one move had triple HP over the default, as I triple HP'd most bosses in my game due to fights that way too short, generally speaking I can finish most non boss fights (before modding) without them even getting a turn and most boss fights within 2-3 turns which was just ridiculous, the game seemed to be designed that the player would just rush through it on low level weapons and with no levelling up whatsoever.
The way I see it, every single boss in the game should be capable of withstanding a omnislash attack, kotr should make fights easier but not finish boss's of in one move. Red dragon I thought at level 54 had the rune blade and omnislash charged up at start of fight, used omnislash which did around 2k per hit, and its dead. The boss after that which I read many people find hard??? I have never struggled on this boss although it does moderate damage it just dies far too quickly like most other bosses so usually only gets 3 or 4 attacks in at the most. This boss has strong magic defense (very strong on my mod) however I still beat it fairly fast with physical attacks, it got 6 turns in and it had quadruple over default hp, a dexterity boost of 30 so it had faster charge. To get level 54 I didnt do much levelling, I did item collecting where I fought battles to stock up on things like ethers and x potions and once I got these to high stock 80+ I then would carry on playing the game, I did a bit of gil saving in junon area, but not what I would call excessive, and this had me on level 54 at the temple of ancients.
So the solution could be to make enemies give way less exp so levelling up is slower and the time you meet bosses you are much lower level but this would then mean getting to the magical level 99 and level 60 points much harder and less likely in a normal game, I do agree enemy AI's need to be improved especially on bosses and of course stronger attacks ones that hurt enough so that you do actually have to heal in battles and even consider fighting on the back row/use sadness, but doing this alone is useless if the enemy gets no attacks in, and the best way to ensure they get attacks in is to boost their hp/dexterity and defenses.