Two long posts got eaten. Fucking 4chan X.
A Space Marine is horribly OP in the lore, and a Thousand Sons marine can solo a planet. One on one, TS marines > everything but Grey Knights. Ahriman is the greatest of the TS, and he has more skill and knowledge of the Warp than any other human barring the Emperor. He was already the greatest sans Primarchs, but then he got the Book of Magnus, and spent 10k years studying the intricacies of the warp, so he's pretty OP lore-wise.
As far as brute strength, he's at least Beta, possibly Alpha class depending on who you ask. If he ever gains access to the Black Library, he could become a new Chaos God.
As for the "Chaos is a tool, deluded Chaos worshipper" bit, he sees the warp as a tool, not Chaos. He was disgusted when they were sent to the PoS, and he stayed to stop the flesh-change out of loyalty to the Legion. Even now, he only encourages Chaos worship in order to acquire more knowledge or artifacts.
Admittedly, some portray him as "Hue hue hue, Corpse-Emperor lulz", but I prefer him as he was in the book Thousand Sons.
Missing quotes, but that's what I get for quick-replying 8 paragraphs of posts.