Aircraft models in Ace Combat X, and modding possibilities.

[LunaMoo]

On my blind eye I don't see any model difference. Textures however are different, definitely lower res in-game most likely due to memory limitations and for that you can just use texture replacement.

You'll find options for that feature under tools->dev tools->texture replacement, you can also read through this thread to understand this feature a bit better maybe see some examples as well. Due to how psp games work it might not always be easy, but it does not have any silly size limits and if you have some graphic skills you can even make completely new, HD textures.

Edit: actually just checked the demo of "Ace Combat X2 Joint Assault" and if the full game works alike, this might be easiest game to replace textures I saw till now, since all of them are proper psp sizes so ppsspp can easily recognize them by texture hash just replacing address with wildcard;p.
Plane textures at least those in-game(demo doesn't have high res ones) might be a bit hard to notice, they're 256x256 size and almost fully transparent with just edges and details drawn, I was lazy and changed color balance of whole texture to quickly test it and noticed that it made all surface reflective, so I guess texture might be applying reflection to everything of same color as some part of it or just to anything which is colored, since it doesn't have much color otherwise.