As long as the cost/benefit ratio was sensible, I'd be in favour of a new build route between Taunton and Plymouth: not for 140 mph which is unnecessary but as a 100 mph railway. Between Taunton and Bristol, I believe speeds are already quite good.
No point building a new line to 100mph standard, could be argued you should build it to HS2 standards even the trains don't run at that speed initially, and certainly engineering it to the max capability of IEP would seem to make sense to me even if they only ran initially at 125mph along with Voyagers etc.
The problem with whole idea of a diversionary route is that its too wrapped with the obsession of the fanatics trying to restore a rail service to North Devon, when your looking at whole concept of a Dawlish diversion, you should be looking at what's best way to achieve that and whether you should also look at as an opportunity to improve journey time, not on trying to restore a train service to Okehampton etc. and looking at building it between Taunton and Plymouth might well have some merit.
Of course at present there isn't much likelihood of any of it happening so its a pointless argument anyway.
On a wider note with regard to railway re-openings, I think there is too much obsession with trying to reopen old routes, yes it may the most cost effective option in some cases but it shouldn't blind the fact that a new route or partial new route may be much more fit for purpose in the 21st century.