Thanks to everyone who responded!
I taught myself a little more about hex editing, and read through Jeff Ludwig's hacking notes for the GBA version, which wound up being a godsend. I've located most of the data I need to edit, now it's just a matter of 1) Deciding exactly how much I want to adjust things, and 2) Actually doing it!
I debated over how to adjust the experience curve. In Origins' Easy Mode, the amount of experience needed to reach any given level is approximately 30-40% of what it was in the NES version/Origins Normal Mode. In the GBA and PSP versions, the amount is FURTHER halved (exactly) from Easy Mode, meaning only about 15-20% of the original's experience requirement. In other words, levels are gained at a whopping 5-6 times faster in this version compared to the original. (Example: to reach level 20, the amount of experience you need is 96,332 original vs. 32,452 Origins Easy vs. 16,226 GBA/PSP. To put this in perspective, gaining 16,226 in the original version would only have you at level 11. Or to think of it the opposite way, gaining 96,332 in the GBA/PSP version would put you at level 35!)
I think ultimately, I'm going to revert the experience curve to the original NES amounts (and continue accordingly for levels 51-99, which of course weren't present in that version). I considered maybe making the required experience a little less, but then I think about how the extra dungeons play into this. I think all the extra experience gained in those dungeons, assuming you play most of them throughout the course of the main game*, is enough to take the edge of some of the grindy-ness of the original version, while still maintaining its overall challenge.
(*This brings me to another thing I want to figure out- when is the ideal time to play each bonus dungeon? It's been a while since I've done a full playthrough, but I recall the pacing being awkward. If you played them too early, you gained so much experience, you made the main game even easier. But if you saved them till the near the end/post-game, most of the dungeons save for the superbosses were a breeze by that point. Anyone have thoughts on this?)
Now that I have a better idea of how healing items are programmed, I'm thinking about how to adjust Ethers (and Turbo/Dry Ethers). Besides increasing their price and/or limiting their availability (especially early-game), I'm also considering decreasing their potency. If I recall, regular Ether heals 50 MP and Turbo Ether heals 150. I'm thinking about dropping those down to, say 35 and 100 respectively (I'd keep Dry Ether as a full-heal, but again, see about lessening how many there are). I haven't thought much yet about how to adjust MP overall, but if I do it will likely be in the form of slightly increasing required MP for spell cast, rather than messing with any of the growth. But then again, maybe decreasing Ether potency won't make that necessary.
And as I said before, I definitely want to beef up those boss fights! I understand the GBA/PSP version already did increase all boss HP (in a sorta-attempt to balance out all the ways the game was made easier?), but it wasn't nearly enough. At the very least, I'll probably double their HP, maybe even triple in some cases. Maybe I'll buff some of their stats too- I want the bosses to be genuinely more challenging, not just take longer due to more HP.
Anyways, thanks again to all of you, and I welcome any input or advice you may have, or anything else I should take into consideration. Ideally I'd like to have a basic version of this patch done within the next couple of months or so (assuming life doesn't get in the way).