Yes, when Par to Newquay goes hourly I believe they should be using a 175 on one diagram, and a 150 on the other.
Don't quite understand how Barnstaple comes into this though?
When you get a service hourly, such as Barnstaple or Okehampton, it does give the commuter some certainty of how long they will have to wait for the next train; not great. Same goes for the off-peak leisure traveller.
Get it to half an hour and passenger increase. St Ives is a good example as there is a rival bus service between Penzance and St Ives.
Somewhere there is research on how increasing train services has a direct correlation on passenger growth. Same research works for roads. The Kingskerswell bypass and half hour Paignton service has made Paignton, Torquay, commuter towns for Exeter. The exact opposite of what was intended. The same could happen to Newquay.