This is the graph of just AV1 preset 12 against X264 Faster preset
It seems like using SVT-AV1 speed 12 wouldn't hurt a 1080p60 twitch stream as much as I once thought
Going from preset 12 to 11 increases the average VMAF score by about 1% and going from preset 11 to 8 increases quality by only 1.4%
X264's Faster preset got a score of 84.24% and AV1 preset 12 got a score of 90.12% which is a decent improvement in this one test clip.
More testing to come.