Wasn't there something about Imperial listening posts picking up transmissions from outside the galaxy, about planet sized orks?
I don't think so...maybe it was just some comic fluff like the bit in the 5th ed Guardsmen about Guardsmen slang terms.
Gorro eh? I thought the whole throttling story happened in one of the many battles for the empire of Ullanor.
Indeed. Apparently that was retconned so the Emperor could be given more time to consider Horus for the position of Warmaster. Still, at least most of the story elements are kept the same despite the differing location.
Afaik the Imperium sent a probe outside of the Milky Way and the Adeptus Mechanicus promptly started shitting bricks when the probe after some time (and a considerable distance passed) sent back signals of Orkish origin.