The following is from David I. Starling and seems to me remarkably clear and thoroughly Pauline:
At this point in the discussion, the topic of imputation arises - not only because it is a notorious point of contention between old perspective and new perspective but also (and more importantly) because of the language and imagery implied by words such as "righteous," "justified," and "condemned." In Rom 3:24, for example, the justification that is accomplished through the work of Christ is conferred on its recipients "by his grace as a gift" (δωρεὰν τῇ αὑτοῦ χάριτι) - language that anticipates the discussion in the following chapter, in which the justification of the ungodly is described as a metaphorical transaction in which righteousness is "reckoned as a gift" (λογίζεται κατὰ χάριν; Rom 4:4, 6, 12; cf. 5:16-15) and sin, conversely, is "not reckon[ed]" against the sinner (Rom 4:8; cf. 2 Cor 5:19).
Imputation, then (or "reckoning"), of one sort or another, is not an un-Pauline intrusion into the doctrine of justification; it is part of the conceptual array that the texts themselves bequeath to us as a framework within which to articulate our understanding of the righteous status of those on whom God's justifying verdict has been pronounced. If we are to follow Paul's lead in constructing our doctrinal formulations, the question is not whether we will have a doctrine of imputation but merely what sort of doctrine of imputation we will construct - which metaphorical credits or debits we will speak of as being imputed to whom - and how much work we will ask it to do within our doctrinal system. If Paul is happy to speak of God as "reckon[ing] righteousness," "as a gift," to "ungodly" people whose record of conduct could hardly warrant this verdict; if Paul speaks of this gift of righteousness as having been made possible by the faithful obedience of Christ, culminating in his atoning death; and if the forensic and covenantal background against which Paul makes these assertions is one in which "righteousness" is language not only for the status created by a judge's verdict but also for the record of conduct with which this sort of verdict ought normally to correspond, then surely, one might argue, we are only connecting Pauline dots, not drawing a whole new picture, if we speak in terms of God's imputing our sins to Christ and Christ's righteousness to us.
From "Covenants and Courtrooms, Imputation and Imitation: Righteousness and Justification in Paul and the Faithfulness of God," Journal for the Study of Paul and his Letters 4:1 (2014): 37-48, here 43-44. This issue of JSPL was devoted to reviewing N.T. Wright's recent and substantial Paul and the Faithfulness of God.