I think May timetable change will be the crunch when diagrams are likely to change with 175s replacing the 158s at Exeter and allowing 150s to take on Newquay hourly. An hourly service to Newquay could require four car trains because GWR may get a surprise in extra customers. The turbos will still be needed for Paignton-Exmouth.
If GWR want to improve the services round Bristol then they need the 158s. Exeter with it's new depot facilities and staff accommodation was supposed to become the centre of the universe for the South West.
GWR sort of get a few extra weeks in Bristol area due to engineering works.
The Severn tunnel route is closed from Saturday 21 May to Monday 8 June, and no trains Fareham-Portsmouth last week of May either.
But the 158 strengthening on Portsmouth route really needs to ramp up on Saturdays from first weekend in May, as the seasonal Saturday 5 big cruise ship changeover at Southampton starts that weekend, where could be 15000 getting off and another 15000 getting on. Although in a crazy piece of planning (as in not do it before busy season starts) chosen to close Bath-Westbury first weekend in May for 3 days.
It is interesting that Saturdays might become the big problem, because as well as traditional holiday changeover in South West (which goes back decades, and was busy in 1950s and 1960s), now need to Strengthen services to Southampton too.
If Exeter crews aren't ready to have lots of 175 workings by May/June, going to very messy with how they cope with both strengthen Devon mainline and Cardiff-Portsmouth services especially on Saturdays