My view is if your going to build an alternate line then you should look the viability of going the whole hog and look at high speed line between Exeter and Plymouth to substancially reduce journey time, and by high speed line I mean an electrified line with at least 140mph top speed which could initially take advantage IET max capability with perhaps 1 intermediate probably Parkway station depending on routing. Possibly it wouldn't have a good BCR but it could be looked at by the government as a commitment to significantly improve infrastructure to the South West.
The problem with Okehampton route is two fold, one is its unlikely to be significantly quicker, and two the diversionary route is a justification by the fanatics desperate to get this route reopened.
Firstly, you'd be limited to 1 fast trains per hour from London still, so for much of the time there would be very few trains on it.
Secondly, at best, you'd get journey times from London reduced by 30 minutes (so at best 2:30).
Third, to get the shortest route (and therefore the quickest) would be across a fairly inhibited area, meaning that a parkway station wouldn't have many passengers and Totnes would lose intercity services (other than those heading to Paignton).
What does it matter if the time spent on the train via Okehampton is longer. Let's say there's two services from Exeter St Davids an hour taking 60 minutes each. Then there's one train per hour taking 80 minutes from Exeter Central to Plymouth via Okehampton. Most people from Central would take the latter to avoid a change during the half of the hour in which it runs, whilst in the other half of the hour they would opt for the trains going via the coast.
In times of disruption adding 15 minutes to your journey time is fairly insignificant when most people would be using the service for over an hour, if not two hours, and existing delays can easily be 30 minutes when one track is used due to spend/high tides.
By looking for a slightly slower, Via Okehampton, route there could be cost savings over what was presented to the Government.