Well, first of all understand that R3E doesn't support trip-screens natively.
All it does is spread a flat image over three screens, it doesn't warp the image at the sides as a proper set-up would.
I honestly haven't fiddled around with the settings much, I'm just happy with the extra screen real-estate and the improved peripheral vision it gets me.
But I'm running R3E at 5780x1080 with the default High settings, and that's it.
I'd rather race than fiddle with sliders for a fortnight.
Screens themselves are angled at around 25 degrees or so (haven't measured).
Just note that you will definitely need to change your FOV in-game when switching to trip screens.
Otherwise you'll end up with the camera right up in the windshield.