That level of tinkering isn't going to get you level of capacity you'd get from changing to a fleet of class 800 (or comparable).
If you had every 220 and 221 possible you'd have 352 coaches, the suggested 5/9 coach fleet would require 304 coaches.
Now tell me which one the Treasury would sign off on?
Even with the lease costs of the 800's being 15% higher the total cost would be lower (unit rate of 100 for each class 22x coach and 115 for each class 800 coach the total is 35,200 for the 22x's and 34,900 for the 800's).
That's before you factor in lower running costs, higher capacity (so scope to sell more tickets), lower staff costs (limited need for staff in a second unit, as there'll be far fewer pairs of units) and coach count for track access charges.
Putting it simply; it is quite likely it's going to cost more and generate less income to not have a new fleet of class 800 (or comparable) at XC if the option is more 22x units.
It may already be the case that ditching the current fleet isthe cheaper option (even with very limited passenger growth expectations).
If that's the case if we were to ask the person at the Treasury who was to sign this off, "what should we do?" How likely is it that they would opt to keep the existing if the new fleet was actually gong to be cheaper than doing nothing?
As I don't have access to the actual costs, I don't know if that's the case, however it's likely to be worth someone investigating.
At a GBR level of it was a class 800 train we could make it even cheaper.
For example if we lengthen 24 of the 36 class 800's at GWR and transfer 10 five coach sets to XC, then GWR have 12 diagrams of 9 coaches (rather that 12 of 10 coaches) and still have 14 sets with 5 coaches but 2 fewer coaches on lease.
Yes I know that 36-24=12 not 14, but as your only needing 4 coaches to extend to 9 coaches you can order 48 mid coaches for insertion to 12 units to create "full length" trains for GWR, that would normally require 24 units with 5 coaches, so that's 12 units which are now spare. However, with a saving of 1 coach per 9 coach train vs a 10 coach train GWR can retain 2 units of 5 coaches and their fleet would still be smaller than it was by 2 coaches.
It gets complex without prices to set a comparison against.
If we assume that GWR are paying 100 per coach and due to inflation that would be 120 per coach for the new coaches.
The total number of new coaches for the GWR upgrade and XC is 302.
GWR agree to pay 100 per coach for their fleet, only because they have 10 extra coaches that's an increase of 1,000, but they have more capacity without paying the 1,200 which they would otherwise have to pay.
The annual cost of the 302 coaches is 36,240, only 1,000 of that is already being paid by GWR, giving a cost per coach of 116.70.
Doing this and given the other cost savings, XC would likely need to see an around 10% uplift in passengers even with a 20% uplift in lease costs per coach for it to be cost neutral.
Obviously that assumes that GWR can make their extra sets pay, but with cost savingsfor running a 9 coach set over a 5+5 train they're likely to be able to make the maths work too.