The journey from Exeter to Newton Abbot is fine. Somewhat tiresome reading that it is a 60mph secondary line, which it is not.
Parts of the line from Aish to Hemerdon should in fact be able to withstand an increase of line speed from 60 mph to 65 or possibly 70 mph. The problem being that higher speeds would not be available throughout due to various viaducts and tight curves and you would be increasing wheel / rail wear, fuel consumption and brake wear for very little actual decrease in journey time and an increase in cornering forces / cant deficiency effects on the passengers.
The rest of the Exeter - Plymouth - Penzance route is in my opinion already running at the maximum comfortably allowable speed so you can forget any journey time decrease through increased line speeds. Possibly the only section in Cornwall you could increase is Nottar Viaduct - Wearde on the 1906 deviation and that would be no more than 80mph instead of the current 70 mph. Which is of course no real benefit to the 15x operated services which struggle to reach 70 mph on that section if they have stopped at Liskeard or St Germans.
