The idea is, if final authorization is given by the DfT. to use the existing infrastructure to the max because if you need to provide any more infrastructure or significantly upgrade it, that would need a lot more money to be authorized. It would also put the scheme in doubt and push it back a fair few years.
The current infrastructure limits you to an effective maximum of 8 trains a day with a journey time of between 42 and 45 minutes each way (assuming the Sampford stop is made), and that journey time variance largely depends on what else is about in the Exeter area.
8 TPD is one diagram and pushing it further than Exeter St Davids is problematic - there is too much else around between Central and St Davids at the projected presentation times and it is better, if the 8 TPD is still to be run, to connect in and out at Exeter St Davids. It also means you can keep it to one diagram, which would be exceeded if you could push the service further. Two units would be too expensive.
If the scheme is a roaring success, then it could attract further investment and enhancement. GWR is rather good at getting funds for improvement works or service enhancements.