It's going into peak holiday season. It is not unusual for the Paddington to Penzance Trains to be full and standing, leaving people behind, right the way through until Truro when it empties out a bit, but remains busy until St Erth. This pushes those left behind onto Plymouth & Exeter Terminators, who then change at Exeter or Plymouth onto the local stopper filling that up.
With the 175s they can double them up for the school holidays and use 5 car IETs where possible to fill the gaps to meet demand by moving them around. Most of the year, 3 cars will be more than adeqate with a few 4 car workings thrown in. As part of the works at Wolverton, they are being fitted with new driver seats and SDO Mods amongst other things. Saltash will still be an issue, but ensuring passengers are in the right carraiges at St Germans will be a workaround.
Once all crews west of Taunton are trained, then Bristol crews can be trained up, which will allow them to run through to Cardiff again, with portion working and a few 5 car 80x thrown in. Diagrams can also be optimised at that point to get the most out of the stock.