The latest issue of NTS (which is available for free online) has this article by Gerald Peterman
Gerald W. Peterman, “Δικαιωθῆναι διὰ τῆς ἐκ Χριστοῦ πίστ1ως: Notes on a Neglected Greek Construction.” New Testament Studies. 56:1 (2010): 163-168.
Of interest to us is his concluding question and statement: “If, in Gal 2.16 for instance, Paul had wanted to speak, not of faith in Christ, but rather of Christ’s faithfulness, why did he not say καὶ ἡμεῖς εἰς Χριστὸν Ἰησοῦν ἐπιστεύσαμεν, ἵνα δικαιωθῶμεν διὰ τῆς ἐκ Χριστοῦ πίστεως? Such wording would spark debate as to whether ἐκ Χριστοῦ indicates the subject of πίστις or its source, but Christ as object of πίστις in Gal 2.16 would be excluded from consideration” (168).