What's the furthest a turbo can get in the direction of Cornwall? They can definitely get to Newton Abbott, but could they get to Plymouth if needs must? One other limitation they have is they are not cleared beyond Exmouth jct towards Axminster, so when one ends up on the peak diagram from Barnstaple-Axminster, it has to be cancelled at Exeter. Is that a clearance issue that can easily be fixed?
On the carbon emissions point, while ideally trains would have as low carbon emissions as possible, if 2+4 Castle sets increased capacity and comfort to the point more passengers ditched cars for the train, there maybe a good carbon offset compared to a shorter train with lower emissions but fewer passengers. Hopefully 175s and other stock improvements enabled by them can improve all of this however.