Greek scholar Daniel Wallace, though critical of the KJV in general, acknowledges that the KJV correctly translates this intensive perfect at Ephesians 2:8. He says:
Read more articles from: The King James Version is Demonstrably Inerrant
“it should be noted that as many faults as the KJV has, it frequently has a superior rendering of the Greek perfect over many modern translations. (Recall that the KJV was produced during the golden age of English, during Shakespeare’s era.) For example, in Eph 2:8 the KJV reads "for by grace are ye saved," while many modern translations (e.g., RSV, NASB) have "for by grace you have been saved." The perfect periphrastic construction is most likely intensive, however. The KJV translators, though not having nearly as good a grasp on Greek as modern translators, seem to have had a better grip on English. They apparently recognized that to translate Eph 2:8 with an English perfect would say nothing about the state resulting from the act of being saved” (Greek Grammar Beyond the Basics, p. 575).