The 8 trains per hour that wil run from Highbury and Islington down to
Crystal Palace and West Croydon using the extended East London Line
might help a bit, but those are going to be 4-car trains as well...

The new trains should also be more tube-like, and in the end the
service should look something like this:
http://www.alwaystouchout.com/img/pr...il-phase-1.png
http://www.alwaystouchout.com/img/pr...il-phase-2.png