Doubled up 5 car sets appear to be a good option for the sort of routes that are operated, where the passengers start dropping off quite quickly from Paddington. The Hereford trains will have lost well over half their passengers by the time Reading and Oxford have been served. I've gone to Swansea on a train that had standing passengers leaving Paddington but there were only two of us in the carriage getting into Swansea. And since the WofE trains started serving Castle Cary, the seats that are vacated there are never sat in again all the way to Penzance and back.
HOWEVER.
What I fear will happen is just what has been done on the Waterloo-Weymouth line. Those who don't know it will cheerfully write about how the 444s are doubled up to Bournemouth, and one unit only beyond. And indeed that is how it was done initially. But over time more and more of the services were reduced to just one 5-car unit throughout. I can't remember the last time I had a unit added at Bournemouth. And the replaced main line units now turn up on all sorts of other turns, including all stations stoppers.
And that is what I suspect will happen on the GWR once the accountants get to work. Initially only "off peak" of course. So standees on a 5-car set from Oxford, and crush loaded from Reading, certainly on Saturday and Sunday. And meanwhile, sets all sat in North Pole depot all weekend, not incurring mileage charges.
The worst is going west from Paddington. Now that the station staff there refuse to put platforms up on the board until 5 minutes before departure, and given that the Reading commuters are more fleet of foot than those heading to Cornwall with luggage, it's already common for those headed for the end of the line to have to stand as far as Reading. It used to work well, the train was on the board 20-30 minutes beforehand, long distance travellers could walk easily to the front and get comfortable, in the final 5-10 minutes when it became the Next Reading train passengers for there would come and fill in. Not now.
It's already difficult to get a seat in an HST at Taunton that has been a popular Penzance departure. The same issue has become an issue at Bristol Parkway on morning trains coming through from Swansea. Once HSTs start getting replaced by a SINGLE 5-car unit to suit the beancounters, who have off-leased the other 5-car unit to save money, it will cause all sorts of grief. And no, I don't trust the TOCs, especially any new franchise given to the highest premium bidder, to do otherwise.