Timothee Besset sent word that Rocket League [Steam] has now been updated to fix some issues Mesa users were having with radeonsi.
You can see the original bug report here.
TTimo was speaking about it in our IRC and it has now released going by this tweet. It was an issue in the game, so a Mesa update isn't needed.
This should give AMD gamers a much better experience on Linux, as you shouldn't get big stuttering or hangs especially at the start of a game. This issue affected quite a number of people, especially when we play with community during our livestreams so it's pleasing to see it finally fixed.
To celebrate, I will likely include it in my livestream tonight and play with the community.