Torrey Seland provides a helpful overview of Ben Witherington's arguments for a predominantly Jewish audience of 1 Peter. I agree with Seland that Withering has mounted an impressive case for a Jewish audience.
Doing some research I found this statement by Craig Keener:
An audience in Asia Minor might consist mainly of Jewish Christians, but Peter’s audience probably includes Gentile Christians (cf. 1:18; 4:3–4).Craig Keener, IVP Bible Background Commentary: New Testament (IVP, 1997).
The type of commentary doesn't allow for footnotes, but Keener is well known to be an authority on first century sources and places. And thus he appears to confirm Witherington's position that these areas had sizeable Jewish-Christian population.
I must confess that 1 Peter 4:3 sill suggests to me a Gentile audience. I'm struggling to see how this can refer to Jewish Christians.
ἀρκετὸς γὰρ ὁ παρεληλυθὼς χρόνος τὸ βούλημα τῶν ἐθνῶν κατειργάσθαι πεπορευμένους ἐν ἀσελγείαις ἐπιθυμίαις οἰνοφλυγίαις κώμοις πότοις καὶ ἀθεμίτοις εἰδωλολατρίαιςYou have already spent enough time in doing what the Gentiles like to do, living in licentiousness, passions, drunkenness, revels, carousing, and lawless idolatry.
Witherington’s comments on 1 Pet 4:3 are all the more therefore surprising as he notes the following concerning this verse:
Notice that all of these vices listed are things that went on at pagan festivals or dinner parties, including in temples. Drunkenness and orgies are forbidden, but notice also the prohibition against idolatry, here called “disgusting” or “lawless [athemitois] idolatry. Pagan idol feasts is a subject that Paul addresses as well at length in 1 Corinthians 8-10, as does Acts 15’s decree articulated by James, and we may see this as one subject for taboo in Revelation 2-3 as well. Second Corinthians 6:14-7:1 is the Pauline form of the same advice: “Do not become entangled in pagan idol feasts and so be unequally yoked spiritually with unbelievers.” The association of idolatry and immorality is quite natural in Jewish polemic, because it all happens in the same venue: the pagan temple.
It is exceptionally difficult to see how this refers to a Jewish audience. Can we suppose that all Jews living in the area’s of Pontus, Galatia, Cappadocia, Asia, and Bithynia, have been engaged in such activities? Or is this just hyperbole? Can we imagine a sizeable population of Jews, even Hellenized Jews doing all this? Engaging in orgies? Idolatry? The question then becomes, in what sense were they still Jewish?
Yes, Witherington can marshal texts from early Judaism that warn of “debauchery” and “idolatry” but what help are these texts in illuminating the situation at hand? Was Peter aware of these texts? Would Peter have referred to all Jewish Christians of these areas the way he does in this vice list? One should also note that some early commentators on this passage suggest a Gentile audience. Thus, there is no consensus or unanimity among patristic commentators that Peter writes to a Jewish-Christian audience.
 Witherington, A Socio-Rhetorical Commentary on 1-2 Peter, pg. 196. Witherington does reference an earlier article of his, “Not so Idle Thoughts about eidolothyton,” TynBul 44, no 2 (1993): 237-54. So one wonders if this advances his case for understanding this as referring to Jews. [If anyone has a digital copy of this article and would like to send it to me, I would be MOST grateful!]
 Testament of Judah 14:2-3
 Testament of Judah 23:1
 Witherington, A Socio-Rhetorical Commentary on 1-2 Peter, pg. 196-7
 See ACCS 11:112. Thus, not all patristic commentators viewed 1 Peter as written to Jewish Christians.
 Contra what Witherington, A Socio-Rhetorical Commentary on 1-2 Peter, pg. 17 appears to suggest.