It's well established that Southeastern do not have depot space for additional units - even the 377s allocated to them are still based at Selhurst. Having said that, would a cascade from South West to South East allow all of the Angel trains owned networkers (i.e. those that have not been retractioned, 465/2 465/9 and 466) to be taken off lease? The 700s could work similar diagrams as the current 5 car 376s, and 458s could replace the 465/9s on longer distance work.
I'm fairly certain that the infrastructure isn't there yet to handle that many 5 car units, and the prospect of training staff on two new multiple units from two different manufacturers does not sound pleasant, but even if one of the two fleets were to transfer it would bring about significant improvements to capacity.
In all likelihood the new SE franchise will probably go ahead and order yet another new fleet, but running the numbers briefly made it appear as if the suggestion might not be as odd as I first thought.