All of which will involves two shunting trips variously blocking either main running line and isn't going to take much less than an hour to complete if you are lucky.
Given that all the trains to Worcester in the peak already HSTs with 10 vehicles, and have been for years, you are trying very hard to solve a problem that doesn't really exist, while creating several new problems. I would vote any day of the week for 20% more standard class seats from the same number of vehicles.
If - and it remains an if at this point - in future some IET formations divide and join at Oxford, that will be done in the platforms in a few minutes - just the same as has happened with Turbos for the past 24 years. Shunt moves out of the down side sidings do not block the main line for an hour, but I assume you know that anyway, so why post nonsense?
Yes, of course the Cotswold Line peak trains to London in the morning and back in the late afternoon and early evening are HSTs, because the loadings at those times of the day need a high-capacity train - but you seem utterly unwilling to listen to anyone who tries to point out the monumental stupidity of running trains with seats for 550+ people, never mind 650 or something north of 700, on this route, and to other places, the rest of the time.
The services going the other way at those times of day, the off-peak services and most of the weekend trains do not need hundreds and hundreds of seats, because there simply aren't hundreds and hundreds of passengers travelling to and from the largely rural areas and two small cities the line serves past Oxford all day, every day. GWR has not run off-peak HSTs to Hereford in recent years because they are what's needed, but because there was nothing else more suitable in the fleet, with just a handful of 180s available in the past few years.
Just because you perceive there to be a problem between Reading and London - not that you seem willing to wait and see what the impact will be of the new 387 services from this week, the extra capacity more IET services with higher frequencies on most routes will offer as part of the big GWR timetable change at the end of next year, with Crossrail to come after that - does not mean that should dictate the shape of the entire GWR long-distance train fleet for the next 30 or 40 years and saddle the operator with monster trains that are simply not what's needed.
Which is why something else got ordered instead.