Narkissos....What is Brown's thesis specifically? I'm not well acquainted with the niceties....
About Ignatius, I don't think this is a mere case of hyperbole in that such people are merely not talking enough about Jesus. Such a failing doesn't justify the utter condemnation of such people as being "tombstones and graves of the dead", whose behavior was more intensely wrong than the teaching of Judaism (cf. ean de "but if" in 6:1b which emphasizes the subsequent described situation). The plain meaning of not even professing Jesus (thereby placing such Iousaismos beyond the pale of Jewish-Christianity) better fits the rhetorical language, and curiously enough the same metaphors of "tombs" occurs in Matthew 23:26-27 to condemn Pharisaic Jews. I also disagree that there would not have been social motivation for ppl to leave Christianity to Judaism which had already "lost its temple and priesthood". I don't think we should necessarily think of thoroughly mystery-cult oriented Gentile Christians defecting to Pharisaic Judaism but to diaspora Jews who originated within the Jewish synagogues, entered into the Johannine Christian communities, but who eventually returned to their "home" communities when the going got rough. Pharisaic Judaism was even in futher dire straits after the disaster of the bar-Kochba revolt, and yet Justin Martyr clearly talks about those who "once professed and recognized that this is the Christ, and for some cause or other passed over into the life under the Law, denying that this is the Christ" (Dialogue with Trypho, 47.4). Here Justin shows that the synagogue issue remained alive into the middle of the second century, that ethnic Jews were still drawn back to the synagogue and ended up denying their faith in Jesus. I think the synagogues continued to be a draw for ethnic diaspora Jews whose families were still in Pharisaic Judaism (cf. the many synagogues attested in Asia Minor) but who themselves had converted to Christianity. I don't think we really can know how serious the defections were, since the historical evidence is so meagre. But considering how Christians were kicked out of synagogues after A.D. 70 and persecuted thereafter (as attested in John 16:2; cf. Matthew 23:34; Mark 13:9; Luke 21:12), there would have been clear reasons for those attending synagogues for social reasons to no longer publically declare their faith in Christ and no longer confess Jesus as the Christ. And then by the 130s, rabbis who allied themselves with Akiva and bar-Kochba would have surely resisted the profession of anyone else as Christ, and Justin Martyr in fact describes such enforced apostasy during the bar-Kochba revolt when bar-Kochba's men rounded up Christians to be cruelly punished unless "they deny Jesus [as] the Christ and blaspheme" (1 Apology 31). But I think this lies outside of the timeframe of 1 John.
Griffith also makes one other interesting observation. There is a very close literary connection between 1 John (especially 1 John 3:4-15) and John 8:31-39. Thus the phrase ek ... tou diabolou occurs only in 1 John 3:8 and John 8:44, while the phrase ek tou theou (which occurs three times in 1 John 3:9-10) is found only in John 7:17, 8:42, and 8:47 in the context of disputes with Jews. The term anthropoktonos is found only in 1 John 3:15 and John 8:44 in the NT, the phrase poiein ten harmatian is found only in 1 John 3:4, 8-9 and John 8:34, ekhein ton patera is also limited to 1 John 2:23 and John 8:41, the term sperma is shared between 1 John 3:9 and John 8:33-34, and the pseus/pseud- word group, occurring 9 times in 1 John is found only in John 8:44-45. The concentration of parallels between 1 John 3:4-15, which deals with apostasy, and John 8:31-59 suggests that those against whom such language is used in 1 John, namely, apostates, are precisely those described in John 8:31 as tous pepisteukotas auto Ioudaious. Griffith suggests translating "those who had believed in [Jesus]", with the perfect participle indicating that they no longer believed in him (compare the past participle in John 11:44). These are people who initially "put their faith in him" (v. 30), but who really had "no room for [Jesus'] word" (v. 37), who "do not believe" in Jesus (v. 45), and particularly, who "do not believe that [Jesus] is the one [he] claims to be" (v. 24). This last statement appears to plainly refer to apostates' rejection of Christian "claims" about Jesus. Since it is precisely this passage (laden with vitriolic language) that is textually linked to 1 John 3:4-15, this seems to be another nice piece of evidence that fits into the story Griffith is telling.