Turn on HD on GT4 PAL

[zero29]

there's also the GS Mode Selector

[VIRGIN KLM]

The 1080i resolutions in PS2 games is just a scalled image to 1920x1080 aka fiasco. It was made at the time TVs had bad (or not at all) internal scaller so people could enjoy it on their early HD screens without issues. The internal resolution or better source resolution is the same on all vmodes, nothing changes, nothing gets rendered on 1080i or 540p at all, I'd stick with 480p.
Oh also, that mode is not available on the PAL version, it's not even hidden, it's just absent in the game's code.