I thought the thrust of Grecian's vision for the Devon & Cornwall area was to reproduce the A30 and A38 in strategy terms. In other words there would be this spine route (A30) off of which would stem the re-instated Plymouth route, which itself would continue over the Tamar bridge as now to SE Cornwall and join up further down.
As icing on the cake, the new Haldon route would enable access to Torbay and from Aller on to Plymouth (still).
One has to be careful with this Cornwall stuff. I heard Paddy Ashdown once confessing that he had started a speech in Saltash by saying 'it's good to be back in Devon'.