Some future ideas for more optimal circumstances on PS2:

• The PS2 GPU memory is fast enough to flush and refill textures multiple times per frame, according to the developers of Metal Gear Solid 2 - who had up to 10MB of textures on-screen at times inside the 4MB VRAM (total, not including memory used for buffers). It may be possible to use similar techniques to achieve better results in this demo.
• I feel better looking textures could be used with the above method and storing these in the main 32MB RAM and sending them to the GPU as needed - the PS2 has a 3.6GB/s memory bus so the potential is undoubtedly there.
• Using recent gsKit tools available on github may also give the code some cleaner syntax and possibly expand the available libraries?
• Different video modes; I would say that a 640x448i mode and a 640x480p mode would be optimal, however 640x540 at 1080i would also be very interesting - a few games used this for a 1080i mode, and this could look great on HDTVs possibly with antialiasing.

Keep this project up, though. It's nice to see demos like this arise on the PS2. I recommend you check out the gsKit "hi-res" sample code for an idea of what you might be able to do - this seems like a relatively simple scene, and one programmer was able to get a rotating 3D teapot running at native 1920x1080, a 1920x1080 background texture and all with antialiasing at 60FPS. Would be cool to see some improvements here from code like that.