I took this, and took the 10 largest textures, all 256 x 256, and shrunk them down to 128 x 128 each.
The before & after difference is visually slightly noticeable:
View attachment 81174
View attachment 81177
But more importantly, I found that it didn't seem to impact performance. So apparently, the number of objects or polygons is more important. Then again, if the game needs to stretch them to render them, essentially creating a new image of the target size, then it might just be doing that before loading them to memory for use, so effectively the textures would still be the same size during rendering (I'm assuming they would only be stretched and loaded once).
zankyou, what do you do to the scale when you're importing these? I'm wondering if there are larger versions of these textures, because next I'd like to try doubling them to 512 x 512 instead. Also, if we shrunk the stage to 50% size, we'd have the same object and polygon counts, but the textures would be covering less area, so maybe the game (if it is indeed just stretching them prior to loading) would think the textures wouldn't need to be stretched anymore, and we'd see the "true" impact of smaller textures.
Edit: attached the file with the reduced texture sizes.