SLs are great and doing them is the best way to practise your technique. Thats how i have become a good player as well. I focused on few tracks, namely vineta k, sebenco climb, ubermall and anulpha pass. I tried to copy those brs shown in the vids and eventaully i was able to do so. But more importantly, i was able to understand the physics of the aircraft; how it behaves in the air, how airbrakes, sideshifts and pitching affect the airtime. That knowledge could now be used to apply it to the other tracks and master them as well. And now i can be fast, really fast on almost any track and any speed.
But here is the BUT:
With SLs you do NOT train consistency, which is absolutely mandatory to perform well in a race and also tournaments.
Thats why i recommended you those Time Trail videos. Naturally, TTs are more related to an actual race and therefor better if you want to be a good online-player.
For example, whats the point of being radically fast on ubermall, when you get stuck 4 out of 5 times in that "tree" after the big bump when you try to boost over that passage? Even one time is too much.
This lack of consisteny is the reason why i often lose/lost to the guy who replied to your topic earlier
Anyway, i hope you consider my words in your training and that they might come in handy in the future!