Virgin Trains do not call at Banbury! The reason Banbury is a restricted stop is because there are already trains to London run by Chiltern (believe me - that is what
W&S▸ said when they set up their website!).
Therefore, one assumes that the Arriva trains will be restricted at Bicester and Leamington (although I am not sure they
have to).
W&S want to increase their services in the future. The Arriva services will restrict W&S's options.
Really, the whole plan is not necessary - there are 3 trains per hour to London from B'ham International with an hour's journey time. Why would anyone want to spend an extra hour getting to London to avoid a simple platform change?
And I am not happy about the choice of rolling stock. Any 90 mph trains might hold up Chiltern's 100 mph trains. Chiltern will want more 100 mph track to allow for the 90 minutes journey, but they'll get held up behind the Arriva 158s (which will probably be late knowing Arriva).