Switching a 5 car diagram to a 2x4 car diagram will result in a big increase in capacity, and releases a 5 car set to strengthen a previous 4 car set diagram, which then releases another 4 car set to help double up another diagram. From my bedroom window (between Bristol and Taunton) I see many more double sets now than I did before.
The use of the 5 extra units would have allowed for (not trying to work out for parts of Diagrams, as that gets complex, how've needless to say would show a larger chance of pairs of units):
Diagram A (was 4 coach) to be run by no.1 of the new 5 coach sets
Diagram B (was 4 coach) to swap with Diagram C to be a 5 coach set
Diagram C (was 5 coach) to be run as 4+4
Diagram D (was 4 coach) to be run by no.2 of the new 5 coach sets
Diagram E (was 4 coach) to swap with Diagram F to be a 5 coach set
Diagram F (was 5 coach) to be run as 4+4
Diagram G (was 4 coach) to be run by no.3 of the new 5 coach sets
Diagram H (was 4 coach) to swap with Diagram I to be a 5 coach set
Diagram I (was 5 coach) to be run as 4+4
Diagram J (was 4 coach) to be run by no.4 of the new 5 coach sets
Diagram K (was 4 coach) to swap with Diagram L to be a 5 coach set
Diagram L (was 5 coach) to be run as 4+4
Diagram M (was 4 coach) to be run by no.5 of the new 5 coach sets
Diagram N (was 4 coach) to swap with Diagram O to be a 5 coach set
Diagram O (was 5 coach) to be run as 4+4
That's 15 Diagrams, which is likely to be up to 60 different trains (assuming each diagram can run A to B, B to A, A to B and B to A again, which isn't always the case).
Although, that is enough for around 30% of the 220's in pairs.
However, even if each Diagram only goes and then comes back that's quite a few extra seats available (I'll count total extra) across the day that's an extra 2,460 seats.
Obviously you have to factor in the loss of the HST sets. That's a loss of 45 seats compared to the 4+4 trains (although that's split by class of 28 fewer standard and 17 fewer first class seats, the latter probably being felt less keenly).
Doing the same (2 trips per day for each of the 5 sets) that's 890 seats "lost" bringing the total of extra seats to 1,570 across the day (minimum).
However that's against (again based on 2 services a day per unit) 25,900 total seats (+6.06%).
However, with around 170 services a day run by the 22x fleets and only having 59 units, it's closer to an average of 3 trips per unit per day which would increase that capacity increase a little, especially if the former HST's didn't previously do 3 trips.
As I said at the start if parts of routes (i.e. are pairs in the core but single units for the outer reaches) are strengthened then that would show a larger increase.
Whilst not significant (and obviously not evenly spread) a sightly better than 5% increase (even when compared against the with HST's, and assuming only two trips per unit) in total seating isn't too bad.
Obviously, this is fairly rough and ready, as I'm not accounting for spares, or which services are run by what length of train, etc. However, it should help illiterate that it appears that the 5 extra 221's will be sightly better than have keeping the HST's.
One final point, for each extra 221 brought into use for XC from now onwards (if that were to be a thing and as it wouldn't have to replace the HST's seating capacity) that would see daily seating capacity rise by 1.9% when compared to the baseline of the seats in use when the HST's were in use (so one extra would bring the total to +7.9% whilst 5 would be +15.5%).