It is calculated under the formula that you have certain space requirements which are not carriage length dependent such as wheel chair spaces, toilets, doors, etc... extra space after those are accounted for can be pure seating taken from couplers and dead space at the carriage ends, so actually has much more bang for the buck than you would expect. That extra 3m is enough for an extra set of four seats and table (1820mm) and an extra row of seats (816mm) with some extra leg room. So thats 12 extra seats per carriage, multiply that by the number of carriages and you get that many more seats than an equal length 23m train.
Thats why NR believes its cost efficent to spend circa £800m on 26m carriage clearance across the network.
The thing is you see...
8x26=208m is pretty much the same length as 9x23=207m.
So that extra 12 seats multiplied by 8 gives you an additional 96 seats, but an extra 23m carriage, that because it's part of an MU set doesn't need any disabled toilets or anything (hence rendering that point moot, they don't need to be in every carriage) and would fit within the same platform length as an 8x26m unit would provide you an additional 70 - 74 seats.
Therefore, all of these new clearances, etc. are for nothing more than an additional 22 - 26 seats per '9 car' set?
Sorry, but for me, creating and clearing a whole new running profile and concept, and not even being able to prove the additional 12 seats per carriage, as previously mentioned, they would need tapered ends, coupled to having less doors per passenger, all the clearance issues, and every other problem associated with the IEP or any new gauge profile (Most other stock is C1, C3 or C4), why should we bother for the sake of 26 seats, out of a set likely to have over 500?
No doubt someone would claim this is a 5% increase in capacity, but to be honest, I could make it a 15% increase in capacity without breaking a sweat.
This 207m long set, make it 230m long, oh look, mummy mummy, this set has 74 more seats than the 207m long set... (15% increase in capacity)