system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40k] The Tau are discovered to actually be humans and join the Imperium what else changes as a result?

So through early diplomatic negotiations between the Tau and the Imperium of Man a sample of Tau DNA is acquired and is discovered to be human and the change in appearance is due to a rather benign disease contracted by the first humans that were stranded on the Tau's homeworld(insert warp travel accident with mild amnesia) and through a simple injection the disease is reversed and the Tau are completely human again resulting in themselves and their tech being welcomed with open arms into the Imperium.

What enemies are in for the most hurt and which couldn't care less? 

Which side would adapt the others tech the most or would we see a 50/50 melding?

Would the injection and removal of the "disease" result in Tau being more susceptible to the Warp?

Most importantly... would this be enough to push humans into being truly the most dominant force in the universe?
gpt
The Imperium's "official policy" is to beware the mutant, the alien, and the heretic. 

Let's say we find out that they are, genetically speaking, fully human. Guess what? We're still going to murder all of them.

Up until this point we have assumed they were filthy xenos. That is not a belief that will change quickly, even if we can undoubtedly prove that they aren't. Do you know what we do to filthy xenos? We murder them. 

Let's say that we fully accept that they are human and that the Imperium's view reflects that. Awesome. Guess what? Doesn't matter. Want to know why? Because they don't accept the God Emperor as their leader and savior, and they have actively fought against his children. That makes them filthy heretics. Do you know what we do to heretics? We murder them. 

Let's say that the Imperium fully accepts that they are human. Let's say that the Tau "come to the light" and begin to worship the Emperor. That should redeem them, right? Nope. Want to know why? Because they don't look or act human at all. That makes them filthy mutants. Know what we do to mutants? We murder them. 

Finally, let's say that we accept that they are fully human, they come to their senses and begin worshiping the Emperor more fervently than the Ecclesiarchy, AND we can make them look and sound and act perfectly human. Surely at that point we should welcome them with open arms? No. They will all die. Want to know why? Because they invent. They make use of science. They manufacture AI. Do you really think we are going to risk entering another Dark Age of Technology? Do you really think we are going to look at their flashy, streamlined weapons and think, "Yup, those things are way better than the weapons that have essentially carried humanity through the worst times imaginable, and continue to do so"? Fuck no. We are going to glass their worlds, murder any stragglers, and make damn sure that absolutely nothing of their technology or culture survives.