The basic point I was aiming to make is that dave1987 always makes it sound as though between the hours of 9am and 4pm and after 7pm on weekdays and all weekend that the only trains to be seen on GWR express services will be five-car sets, never mind that, as you observe, a lot of the 2x5 formations are likely to be coupled throughout the day and that there will be a mere 28 nine-car sets available across the 801 (21) and AT300 (seven) orders - which, of course, will not be spending much of their time in the sidings.
...let me get this straight IEP is expensive and has to provide a set number of diagrams per day, so GWR will leave them sat ideal when they could be using them to provide extra capacity on known busy services.
It has been pointed out that there are enough 5 coach sets to allow all the HST's to be replaced with pairs of 5 coach sets and still leave 6 sets (more than enough to also replace the 180's).
Unless you have something more scientific than what you think will happen, then the only way we are going to find out what GWR will do is to wait to see.
I have shown that GWR don't HAVE to run lots of short sets to provide the extra services, so now the view is that GWR will deliberately run short sets when they don't have to.
Virgin trains have 9 and 11 coach 390's which do I mostly see off peak in use? The 11 coach sets, in fact one of the few times I've seen a 9 coach set off peak it was sat ideal in the sidings. If Virgin do that, why would GWR necessarily be any different.
Do you have any examples of TOC's who deliberately (I.e. they have suitable available stock) run trains full and standing? As that is what you are saying that GWR will be doing.
The reason that TOC's (with enough resources) wouldn't want to do that is that by doing so there is no space for growth, if there is no room for growth then more people can't (or at the very least won't) use the trains, if nor people aren't using the trains then the TOC's won't make more money. The failed ICWC bids assumed growth of 5% per year, the TPE winning bid (First) and the Northern winning bid are both providing a lot of improvements based on the fact that the number of passengers will increase.
The way TOC's work now is by relying on growth, they are not going to curtaile that by leaving units ideal in sidings that they are paying for.