Civilizations with Transwarp repeatedly go faster than Warp 9 aka the Volm and the Borg, Also Slipstream is a thing from Voyager that can cross the galaxy in seconds.
It would have to be this version of the characters we know to have any hope of surviving tbh. Also I think steroid injections must be mandatory in the mirror universe because everyone is fucking jacked.
Would be better if it was the mirror universe Terran Empire the Federation is too nice for 40k but the Terran Empire could still come off as good guys considering how shitty 40k is.