@Klomster Actually, marines really aren't all that fast, in terms of reaction time. They've got higher quality sensory input than the average human, but nothing they get actually increases the speed at which they process that information. The only thing is training and conditioning, and that only gets you so far before you hit the physical limitations of relying on a biological sensory system, which humans are already pretty damn close to to begin with.
So you still see Marines as just large humans in armoured suits then?
Your first part clashes with your second part, in my opinion, as it is their very training and conditioning (a good deal of it hypno-suggestive) that serves as the very thing which - partnered with constant training for warfare and their power armours sensors - makes them so fast when compared to humans; the whole point of the Black Carapace and the multiple sockets all over them.
Combined with the fibre bundles, and even without the armour, Astartes are just purpose-built weapons of war with their training regimes and conditioning doing mentally much the same as you've suggested with the Ogryns, the only difference being that a Marine can judge the best means and alternate routes to survival.
If we accept what you say, then why aren't Orks shown as being faster than Marines? They're much like green Ogryns, especially the larger ones.