Well, for what it's worth, here's a quote on "Space Seed" from the original 1981 edition of Allan Asherman's The Star Trek Compendium, predating TWOK: "The character of Khan is one of the most magnetic ever to appear in a segment of Trek."
On the other hand, in David Gerrold's 1973 The World of Star Trek, "Space Seed" is barely mentioned. There are no shots from it in the photo insert section, and in his critical analysis of the series, where Gerrold discusses the best and worst episodes, he makes no mention of it either way. So it didn't stand out from the pack for him.
Similarly, it's not mentioned in 1975's Star Trek Lives, a nonfiction book about ST fandom. Lives contains an overview of the fanfiction of the time as well, and while there are mentions of fanfic followups to "Amok Time" and "The Enterprise Incident" and "Mirror, Mirror" and "Is There in Truth No Beauty?" and Finnegan from "Shore Leave," there's no indication that anyone was doing fanfiction following up on Khan. (For that matter, when fanfic authors Sondra Marshak & Myrna Culbreath went pro with The Price of the Phoenix in 1977, they created their own superman, Omne, when they could potentially have used Khan instead.)
I'm trying to remember what I thought of "Space Seed" back before '82. Hard to sort that out from the subsequent 30 years of memories. I don't recall whether it stood out for me more than other episodes, and if it did, it was probably mainly because of the Botany Bay FX shots. Actually, come to think of it, I recall a time -- even after TWOK -- when my memory of the specifics of the episode's story was somewhat vague.