You do need it, that GTA IV black blocky object/interlacing effect completely ruins the game for me, and for others that have complained about the effects.
As for the rest, I've only lowered to those settings after starting the Trevor missions.
The FPS can be nice in the city, but go into the wild or those areas and they take a massive nosedive! I know because at first I WAS running SP with most things max.
Then I found out how nice good FPS is
Maybe my card is just **** but I cant get good FPS with textures on very high, not even texture high-shaders very high as that also becomes almost unplayable..
Yeah soon as i was in the desert with trevor my fps did take a dive to 45fps at the location of the bikers before u chase em but a bit of tinkering and i got 60 back without dropping anything really just applied the adaptive vsync trick and seemed to work and maybe dropping one thing down i cant recall what it was but it was minor. Oh hang on i had msaa on 2x and txaa on, i took those off for the trevor bits thats what gave the boost. Can put em back up now since im back in the city tho. Even tho fxaa on is rather good tbh. But you dont have aa on so dunno how you cant get very high textures on. Ah well, maybe ill post my settings up later and you can try em if you want see how you go with em. Ive adjusted a lot to get the best visuals vs performance so its taken awhile to fine tune.