I know this a bad thread to bump, but had to lol at this horrible mismatch. Horus vs Khan is like a grown man versus a particularly aggressive ladybug. He could literally flick Khan's head off of his shoulders before Khan could form a thought. Just the speed difference is a joke. Guilliman, for example, in The Unremembered Empire, saw supersonic bolter rounds as moving in extremely slow motion from his perspective, and in Flight of the Eisenstein Dorn moved so fast that not even Space Marines could register his movement. And based on showings, and dialogue between characters, Horus was well above either of these two in one-on-one combat.
An actual proper contest would be Khan vs one of the Lucifer Blacks or Geno Five-Two Chiliad.
Well, I was pointing out the sheer stupidity of this match, and how unbalanced and obvious spite it is (by using words such as "horrible" and "mismatch"😉, but also tried to raise the point that there are actually reasonable characters you can pit against him for a good thread. Khan isn't on Space Marine level, but there are a number of 40k and HH-era characters who would make a good match for him. For example, guys like Harlon Nayl would be an interesting fight with him IMO, depending on the match stipulations.