GT4 1080i on PSX despite D-Terminal port limitations?

I'm pretty sure that GTA4 1080i mode outputs odd and even frames in such a way there is extra detail, it's not just outputting identical 540 line screens repeatedly. If you can render 960x540 fields at 60fps then it will look definitely look higher resolution than 640x480p. However you probably would probably drop the framebuffer from 32 bit to 16 bit, so you may prefer 480p. If you can't render each field at 60fps then interlace needs a whole lot more ram, so it may output 640x540 but interlacing would still give you a higher vertical resolution.

Emulators are notoriously bad at outputting interlace modes though, so for PCSX2 then I definitely wouldn't bother.