Honestly, when I read this manga, I noticed the author provides explanations that aren't in the hentai doujinshi, giving a logical backstory for all the characters. I think the level of detail is amazing. If you only read the hentai doujinshi without reading this manga, it feels like you're really missing out.
For example, the manga explains why Imaizumi's physical stamina keeps increasing in later chapters of the doujinshi—to the point where he overpowers them and lasts longer—it's because he's been working out. Even Yukina is often teased by the others now because she tends to finish way too fast.