They'd be taking more sets than they'd be losing with the mk3s going so there should be a benefit with 168s being freed up from Birmingham duties.
Also 165 can double up on Met line,
168 can double up on Oxfords.
However still subject to final approval from Dft.
Also 5 car Mk5 isn't enough capacity for when on Peak or Weekend services, hence Chiltern looking to make most likely 6 or possibly 7 coaches.
So services will need to go express from Banbury, like before Covid 19.
Even a 6 car mk5 has less capacity than a 6 car Mk3, but is better than a 5 Car mk5.
Obvious will be to run hourly services from Banbury to London Marylebone, to connect with the Birmingham to London services in which mk5 will do.
However as Tpex only ran as 5 car fixed formation, 6 car still needs testing,
As each time you add or swap vehicles,
Or make to 6 car, Mk5 sets will need a complete software reset on them,
Hence only been used as 5 car, in the past on Tpex and being programmed to do so.