I was looking through youtube and saw a video of writing Gregg Shorthand at 300 wpm. It sounded like he was promoting the use of using ONLY brief forms and making abbreviations, using the Gregg Shorthand system, making your own up. It's fine, but you can see if you learn Gregg Shorthand properly, use the brief forms assigned and maybe a few extra like Charles M. Swem did, you can get up there without memorizing all those abbreviations.
Okay, just looking it up, American's use about 10,000 words (high school graduate and it goes up with each degree and specialized field of study). I don't think it would be easy to memorize even half that to write 300 wpm. Even if you just memorized brief forms for your specialized study or interest, that's a lot of words. I guess if you wanted to do that, you could go to Angelfishy Gregg Shorthand and learn the "5,000 most used Words in Gregg Shorthand". Make those abbreviations (take out the words we no longer say) and add to it.
"Gregg Shorthand had achieved the most extraordinary success ever attained by any system in the history of shorthand." John Robert Gregg, 1922