Looking good! Is the sound as good as stock (or maybe better, considering the low volume crackling we get)?
Hoping to give this a shot soon, as soon as I figure out rooting on 9.0. I saw a thread about it, but nobody answered my question there, and light digging through threads didn't turn up an answer, so trying this kernel will have to wait a bit. Makes me miss the HTC root days, when finding out about root was a simple yes or no situation... lol.
Sound seems to have the same issues as stock - slight popping sounds on audio playback, and loud popping sound recorded at start of any audio recordings, but I have a strong feeling these have everything to do with the tfa98xx code razer modified.
I suspect Razer isn't muting the inputs/outputs of the chip while it's being brought out of sleep/reset when audio isn't playing.
I should be able to merge in the latest tfa98xx code from CAF with the few necessary tweaks to fix the popping.
I'm not sure if this is fixed on 9.0 or not, since I'm not running 9.0 yet (I will when src is released).
Also, off topic, but to answer your question rooting 9.0 seems to be fairly straightforward - if you're already rooted on 8.1 you can just take the OTA before rebooting use magisk manager to "install to inactive slot" and it'll patch the freshly installed 9.0 partitions.
If you're not already rooted, you should be able to just flash someone's pre-rooted boot.img and be good to go. I won't get into detail in that here since I'm sure others have a better guide than I do.