It's very circular progress, not linear. First, there's optimizing every single movement, which makes a maximum of 3-4 seconds (mostly less) gained after 6 full hours of TASing work, which usually needs to get redone again until absolutely perfect. I was on lap 2, about to do a mylestyle when I realized I'd have a shitty time trying to get a mushroom on pure luck like I did last time. the only thing that can manipulate an item when you already have one in reserve (the blue shell held behind the kart) is kart movement, so I'd have to mylestyle over and over again until I ended up with a mushroom by chance. (a mylestyle takes at least 1,000 rerecords. the latest one I did took 1,700.)
However, a new trick was discovered and used to successfully skip a lap. This has great potential to either skip one or two (I'm unsure yet) laps at the beginning, so I'm going back (saving all prior progress in a separate .m64 if I fail to pull off this new stunt) to see if more time can be saved. It might lead to a completely new new route.
My hope is that no future course takes this much time to do. So far (yet) I know of no other trick as difficult as the mylestyle, however that could easily be proved wrong, especially as Weatherton, myself, and the mariokart64.com people devise exceedingly diabolical shortcuts.
So progress was slow, slowed again as I experiment with this new discovery, and will continue to be slow. (Luigi's Raceway is supposed to be the easiest of all the courses =D) The end result will, I guarantee, be even more fantastic than the 1st version.