Over the past several days I took the time to create a human theory TAS for any% 8-4 and any% 8-5. The time for 8-4 ended up being 10:55.154 and 8-5 was 11:30.912. I kinda doubt that humans will achieve sub 11 for 8-4 bc there are some things that the TAS does that are very tricky, precise and risky to go for in RTA runs. I don't doubt that humans will get sub 12 though for both 8-4 and 8-5.

Here's the link:

For those who want to compare against the TAS, I timed the TAS via Livesplit while the TAS was being recorded, I will make a resource of them.

An update on the best possible time for humans:
@TheRudyyyTheRudyyy found a new skip where you use the invisible blocks at the flag in 6-2 to jump over it and get to a Bowser fight past the castle and flag, when grabbing the axe you will die and warp to 7-1, allowing the player to skip 6-3 and 6-4 (which was played in my speedruns of 12:26.138 for 8-4 and 13:04.591 for 8-5 | and played by the TAS). This was used (and can be seen) in his recent world record time of 11:04.983 for 8-4, and 11:43.700 for 8-5.

From what I can gather, this LOSES time in 6-2 alone, but saves around 48.653 seconds in the full RTA speedrun. Making the fastest human theory time around 10:06.501 for 8-4, and 10:42.259 for 8-5.

Those times are rough estimates, I highly doubt that's the exact time humans can get, I will make a new human theory TAS soon with the shortcut in it.