What on Earth do you mean "same level" as other Chapters?
Could you provide some sources?
The 7th Edition GK codex states, " ...Grey Knights stand against the greatest threat Humanity has ever faced. They are the champions of reason, order and righteousness, holding back Daemons and the scions of the Dark Gods."
"[The Emperor]knew that of all the Imperium’s many foes, the daemonic was the greatest threat, and so he created a brotherhood of incorruptible warriors to fight Daemons. These were the Grey Knights, and they would stand as the Emperor’s ultimate weapon against the Gods of Chaos."
There's a reason the GKs get the fanciest and shiniest toys.
Can you give one citation of them being described as bigger, faster, tougher or stronger than your common-or-garden SM? They are not born, they are made, just like any other SM.
Their ability to combat demons comes from the fact the purity of their souls means they are anathema to demons, have a staggering mental strength and force of will, as well as having the ability to use the powers of the warp against them. Marry all that to artificer armour engraved with pentagrammic and hexagrammic wards, master crafted force weapons, and blessed ammunition embued with power from the waste products of the golden throne. That's how they can stand toe to toe with demons, not by being any bigger or stronger than an IF or UM.
You also seem to forget that while demons can warp reality somewhat, they are still bound by the laws of physics when manifested outside the warp.
In addition the Emperor had no hand or part in the creation of the GKs, be it their initial founding, or subsequent choice of geneseed used to found a full chapter.
If you think otherwise, sorry mate, but you are the one showing your ignorance here. Malcador assembled the first GKs, or knights errant, not the Emperor. They were presented to the Emperor by Malcador of course, but they did not become a proper chapter until after the Emperor ascended the Golden Throne. Whoever it was that gave them the specific task of fighting demons and selecting the appropriate geneseed for the fledgling chapter, it wasn't the Emperor. He had other things on his mind by then