I found this article called The Influence of the Septuagint on the Greek NT. Here is the concluding paragraph:
J. Ross Wagner writes: My own close examination of the wording of Paul’s quotations and allusions to Isaiah in Romans supports the consensus view that Paul cites a Greek text (or texts) of this prophetic book. In most cases, Paul’s Vorlage seems to have been nearly identical with the Septuagint version of Isaiah; at times, Paul’s interpretation of a verse clearly depends on the form of the text distinctive to LXX Isaiah. In some cases, however, it appears that Paul has drawn his citation from a Greek text that reflects efforts to revise LXX Isaiah toward a Hebrew exemplar. Although I have given full consideration to textual evidence provided by MT, the Qumran finds (biblical MSS, pesharim, and quotations in other documents), the Targum, and the Peshitta, at no point has it been necessary to suppose that Paul has relied on a Hebrew or Aramaic text of Isaiah. This does not prove that Paul could not read these languages, nor does it show that he knew the book of Isaiah only in Greek. It does suggest, however, that Paul was intimately acquainted with a Greek version of Isaiah much like the LXX and that he apparently did not hunt down and exploit the textual variants in other languages as he interpreted the book.The next stage is to compare the Peshitta rendering of the Greek texts in Romans that are quotes from Isaiah. It should be very enlightening.