Though it would be the result of an order for me vehicles, if I'm understanding correctly, so not like there's nothing in it for them.
Indeed, whilst the number of coaches at GWR would be the same (605), they are in a different configuration.
What it allows is 24 units to be transferred to run XC services and so increase capacity (even if they stay as 5 coach sets).
As such that's, as an absolute minimum, an order for of 120 new coaches.
To replace the full XC fleet would need a further 28 sets, ideally with a mix of 5 and 9 coaches, even with just 3 longer sets that's a further 152 new coaches (272+ new coaches), with the potential for a follow on order (say if XC unlocks growth by providing a higher level of seating per train and/or if GWR want to further grow their fleet and/or LNER wish to extend some of their sets to allow then to retire the Mark 4 coaches as well as to cater for additional growth).
That's likely to be quite an attractive proposition. However, the new fleet only has to match the 80x's in capacity and only needs to work work with them in service if they are 5 coach sets. As such it could be that if another provided the 9 coach sets (which ideally for fleet size would need to be 100 coaches worth, so 12 sets with 9 coaches, being 108 coaches).
Such an order would reduce that potential order from 152 to 80 (although more likely 188 to 80, if the XC fleet is getting 12 sets with 9 coaches.
Of course, if it's a case that the 9 coach sets are cheaper, it might bethat they are cheap enough to use them to relocate 39 of GWR's 5 coach sets to XC and replace with new 9 coach sets from another supplier. In such case Hitachi have no new coaches to build, which whilst they could be difficult over renegotiate the deal, they are likely to have messed up quite significantly to have lost the order for lengthening their sets so that they are left with just reallocate part of their fleet is all that's on the table.
Also, I'm lead to believe from what's been said that the diesel engines have been struggling to cope to provide the required number of services, if that's a cost risk to Hitachi (even ifit'snot an issue yet, it's a risk which maybe an issue in the future) then creating a hybrid fleet may help them out.
In your calculations for replacing 5+5 with 9, have you checked how many GWR services are scheduled for single cars? Is 69 (less spares) enough to fulfil all their diagrams?
Short answer, no.
Although having asked AI it suggests 55 to 60 diagrams (due to seasonal variations) with around 1/2 being 5 coach sets.
Even taking the upper number and allowing 15% for spares and maintenance that's 69 units required, whilst that's higher than the 64.5 full length equivalent, the number of units does equal 69.
Now whilst that will mean more services run as 9 coach sets as opposed to single 5 coach sets, the cost for doing so is reduced compared to buying more 5 coach sets.
It's also likely to facilitate growth in passenger numbers and so it's got a reasonable chance of covering those extra costs.
It's also worth noting that there's been calls for more of the Devon and Cornwall services to be 9 coach sets rather than 5 coach sets on here. Whilst these might be from (say) the usual suspects, that doesn't mean that:
A) they are necessarily wrong
B) even IF they are, the cost of doing this is much reduced over other options (in that overall coach numbers stay the same) and so the risk is lower
Even if there's a need for a few more sets (so more than 9 sets with 5 coaches), all that means is that the order for GWR is a bit smaller and XC's order is a bit larger (I.e. if GWR have 9 more 5 coach sets, as opposed to 5 sets with 9 coaches and so don't order 20 of those new coaches, XC would then have to order 20 new coaches - but they might not need to be