I dont know if somebody already came up with this but I have a Suggestion. How about running the Mario Categories without the Intro Sequence but the timer starting at a time where you starting Gateway 1 at the same time as if you would run with the Intro?

To set up the timer I compared the length of the blackscreen in a normal run and if you click on 'Play this File' after you saved and ended at Gateway 1 in the pause screen. I dont know the exact seconds but the blackscreen in a normal run is a little bit longer (about 7s) than after saving and ending in Gateway 1 (about 6s), which is the reason why I set the timer accordingly.

I also looked on the best possible time you can have a the start of Gateway 1, which is, as far as I know, 4:57m. And if you subtract the length of the blackscreen (about 6s) of this, you come to the specific start time of 4:51m.

And this variation would also facilitate the grinding of the Categories, because you do not have to watch the Intro Sequence over and over again, which would cost way less time.

The only thing to remember is: To use it after a failed run again, you have to go back to the title screen through the "Home"-Button.

For illustration purposes here a video of using this variation: