Get an easy-to-drive setup and go from there. Don't change anything and just improve. When you are ready you'll start to see flaws in the setup by evaluating the performance of the car in certain areas.
For example, when I drove low 1.35's, but couldn't quite get under 1.35, I noticed that I lost most time (relatively) in the turn after the straight and the turn to the right before the last turn, so I adjusted my tires to make right-turns easier. After a while I found that I lost too much speed in left turns and switched back to a more balanced setting and voila - 1.34.91!
I'm currently using a very stable set-up which might carry me to 1.33, but the improvements are very gradual and steady (Today I improved my PB from 1.34.63 to 1.34.62
)
Anyway, just try not to mess with your setup too much. A setup is just a tool, eventually you'll start driving faster regardless of setup. Many people will tell you that a setup does not improve your skills, it merely allows you to think differently about your own weaknesses and develop techniques to either turn them into strengths or to hide them in such a way that they become neglible.
Gear ratios are mostly 'fixed' since you want to reach the engines maximum revs on the straight in the final gear. Some changes can be made, but most of the setups on Inferno are similar or exactly the same, I wouldn't worry too much about them.
As for improving laptimes; for me it seems to be very gradual below 1m35s.
I have actually made a video of my own PB and the WR side-by-side, and there seems to be no particular point in which I lose time to MacEST, however in the end I am more than a second behind! To me this proves that very slight improvements which will come as time progresses make all the difference, just keep practicing!
