Thanks so much for finding this, it is something that I have been thinking about a lot in my recent studies. I am pleased to see the whole main line to Penzance covered, but surprised it does not include the branches, even if only the Paignton line for a start.
The routes described would provide full employment for the 165/1 and 166 fleets, enabling a modernisation and standardisation for the core routes concerned, hopefully with some service enhancements.
For a start, the 166s would be well suited to an hourly Cardiff- Penzance service, and the three car 165/1s would provide a step change from the current mixture found on the South Devon locals taking in Exmouth, Paignton and Barnstaple.
The 150s could return North to join the rest of the fleet in Manchester, while the pacers could then be retired.
Of course if gauging turns out to be a big problem and future homes are limited, then the DfT may well end up decreeing (reaches for hard hat) that a large part of the fleet - at least the 165s- join their 75mph cousins on the already cleared Chiltern network and their 168s are released to the more width restricted routes. Just an idea if all else fails.
Sorry, but just can't see where you get the idea 166s would be suitable for Cardiff-Penzance, because if the idea is to use something more akin to a 158, then a 166 isn't that train. It is a tinkered-with suburban dmu, still with mainly 3+2 standard class seating - a layout which is being criticised in another thread about use of 150/1s in the West Country. There is more face-to-face seating in bays but the seats fitted were never intended for long journeys. Same considerations apply to using them to Portsmouth - and there you would surely have to be looking for a four-car formation of some sort.
Unless and until someone bites the bullet over carrying out the modifications to coolant header tanks and braking needed allow 2+2 seats to be fitted in the 165s and 166s (and vestibule doors to minimise heat loss when the exterior doors are opened at stations at this time of the year) they will continue to leave a lot to be desired when it comes to journeys of much more than an hour.
The gauging issues are, I'd surmise, pretty minor on former GWR routes and, as I have said before, a 166 has been to Weymouth in the past on an excursion, so I'm not sure why it is on the list of routes. From the list it is pretty clear that the idea is for the 16xs to take over the routes radiating from Bristol, releasing 15xs for both the West Country and other operators. If XC is then wired north of Bristol early in CP6, you might then free 16xs from Gloucester/Worcester services to move further west.
Having done some sums, I believe FGW has 150 16x cars in its fleet, not 124, formed as 21x3-car 166s, 16x3-car 165s and 19x2-car 165s, plus a spare 165/1 driving car from 165115, the set involved in the Ladbroke Grove crash, with the other two vehicles scrapped.