There is no higher bitrate limit for Partners. I am one.
8000kbps is simply a word-of-mouth, entirely unofficial "known good" value that mostly works.
There is no technologically-enforced upper limit to how much bitrate you can use. (Staff MAY come in and ask you to turn it down.)
I have successfully tested with up to 12mbps, but as you go higher, more people will receive Network Errors and player blackscreens.
Your stream may stop being replicated if the ingest server is unable to process your incoming bitrate. A partial will end up with the Source option being removed, as the ingest/replication/transfer stack failed, but the ingest was able to feed into the transcode stack to the point THAT feed can still be replicated and transferred to the local video delivery CDN servers. It isn't Twitch 'turning off Source'.
Audio bitrate IS factored in.
The difference between 320kbps MP3 and 128kbps MP3 is significant. 320 and 128 AAC (which OBS uses) is mostly un-noticeable unless you're listening side-by-side. I've run down to 96kbps and no one noticed. 160 is as high as I'd bother going on audio.
The drawbacks to high bitrate:
Your viewers WILL receive more player Network Errors and have to reload the stream.
Unless you are a Partner with guaranteed transcodes (quality options), running at a high bitrate will reduce the accessibility of your stream. The higher bitrate you stream at, the fewer people will be able to watch smoothly. Running at a high bitrate without guaranteed transcodes is MASSIVELY shooting yourself in the foot; to grow, you need to make sure that ANYONE coming in will be able to watch your stream, before worrying about wanking over 1080p or 60fps video.
Number-wanking is hands-down the BIGGEST newbie-trap. I fell into it myself at the beginning. A 720p30 stream at 2000kbps with good mic audio and game/mic balance will gain and retain FAR more viewers than cranking the dials to 8000+kbps to satisfy the number wanking "must have 1080p60!!!11!!" that many new streamers do.