I think it is- but I don't see (even with 26m cars) a 10 car (2x5) IEP formation having as much capacity as 12 car (3x4) 365, even though the latter will be shorter.
A pair of five car IEP trains will have 500 standard class seats and 118 first class, assuming that the specification remains the same as that announced a few months ago, while a 12 car class 365 train has 735 standard and 72 first so that is quite a big drop in seating capacity.
However if the composite vehicle in the IEP were to be changed to all standard accomodation, then that would bring the number up to 58 first and 560 standard class seats.
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321s could also head north as there are the 321/9s working in Yorkshire.
To do what though? It is true that Northern are taking on the five class 322s for operation on the electrificed Yorkshire lines, but this is to provide additional service reliability on the Leeds to Doncaster route and to displace a couple of peak time DMU workings on the West Yorkshire lines. Without further electrification of MML and/or TPE North, which would be a mouthwatering concept, then there isn't a need for many (or any) more EMUs working out of Neville Hill.
As
Skimble19 says, I think it will take a combination of 317s and 321 moving to NXEA alongside the introduction of new Crossrail stock on the inner suburban services to see the demise of the 315s on the Great Eastern.
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At the last cascade, the following is what ought to have happened:
London Midland order enough Class 350/1s to fully replace both the Class 321 fleet and provide a increase in seating capacity.
The former London Midland Class 321s would then be transferred to National Express East Anglia in order to free up a equal number of Class 317/6s.
These former National Express East Anglia Class 317/6s would then transfer back to First Capital Connect with the existing FCC Class 317 fleet being refurbished to the Class 317/6 standards.
National Express East Anglia would of ordered enough Class 379s for both the Airport work and also to use on the newly speeded up Cambridges by using one of the existing Stansted Express paths, these express workings would either be Liverpool Street-Tottenham Hale-Broxbourne-Bishops Stortford-All stations to Cambridge, Liverpool Street-Tottenham Hale-Broxbourne-Cambridge or just Liverpool Street-Cambridge.
The First Scotrail Class 380s would not been ordered, both Scotrail and TPE going for 4 and 5 car versions of the Class 350 (The 5 car Class 350's would be equal to the Class 444s used by South West Trains).
If only things had happened this way, it's a very good plan
Ajax103.
However I think the problem with ordering further 350s in their original form is that the cab ends no longer meet crashworthiness standards, so there had to be a revision in the form of the 380s (Although presumably the 20m bodyshells of the 350s could still be constructed without alteration). Just what I've heard reported anyway.
Note that the 350s have 20 metre bodyshells, while the 444s and 380s have 23 metre bodyshells: If it were me, I would have ordered a fleet of four and five car 380s which would be more in keeping with the long distance regional (Bonefide "Intercity" in the case of Manchester to Scotland IMO) as well as with the 185s that would have been replaced on TPE.
Apart from length they aren't much different from your Turbo's 
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