That's why I rush to do the seasonal events before all my PSN buddies. If not the track becomes a mess of ghost lines. It's so chaotic

And I use the racing line on Seasonals, so I need it on.
A good way to avoid this is just being faster than them

I just have problems with one of my PSN friends thankfully, so most of the times I don't have to deal with the "track spaghetti." (as I call PD's dumb idea of displaying your friend's racing line)
As for racing with no HUD, I don't use the map and driver list since these things are useless (and so is the rest). Unfortunately, due to how dark cockpits are, I have to leave the rest on. And then there are cars with faulty or missing gauges. It's so annoying. In GT5 I could keep everything off all the time since it was way easier to see. It felt so great when you relied on the car's gauges. In GT6 it's no longer possible. It doesn't necessarily make you faster, but it is perfect if you want the maximum immersion possible. And that's what I prefer.
As for the ghost, having it on at least let's you examine your friend's moves and judge if you can take a turn faster. In my case, when I turn it off, and then turn it back on, I realized my friends are way ahead of me. When I have it on, I can keep up with them easier.