Unless you are not just travelling north from London, of course.
Please don't condemn Wolves and Crewe (and S Wales, Salop and E Midlands) passengers to "5-car 110mph units" for their journeys to Wigan, Warrington and other stations to Scotland. The Pendolinos are currently the only trains that offer this service and are ftb most of the time.
The "344" concept takes the 444 design from SWR and makes it an OHLE train (see 350/450).
In the unmodified (SWT) layout they held 299 seats. That's lots more than the current 221's (208 seats or 144% of capacity).
Even compared to a 9 coach 390 it's ~64% of capacity and ~51% of the 11 coach version.
However, where there's sufficient demand they would be run in pairs where they would give a capacity increase over the existing 390's.
However what needs to be remembered is that such a unit would be lighter and so therefore have lower costs (I.e. track access charges). Likewise if the design principles of the Desiro City were used to ensure that maintenance tasks were as easy as possible, that would also bring those costs down too.
With such cost savings the cost of running full length trains falls, making that more likely than when compared with the current stock.