A few observations on assorted posts in this thread.
Going back to the beginning, there is no pressing need for FGW to improve its Banbury service, never mind provide lots of fast services to/from Paddington. The Banbury-London market is more than adequately served by Chiltern - the opening of the Crossrail tunnel may make Paddington a much more attractive proposition from 2019 in terms of fast onward travel into central London but until it actually opens, it's hard to know what its impact may be.
Passengers from Banbury to Oxford and Reading, the two key intermediate stations between Banbury and Paddington, are already well served by XC with 2tph per hour and passengers to/from Didcot have plenty of options with some direct services or changing at Oxford. Tackley, Heyford and Kings Sutton are all small villages unlikely to generate a great deal more traffic than they do now, never mind that Chiltern already offers London trains at Kings Sutton and Heyford residents just have a short drive to Bicester to catch London trains.
A Banbury-Didcot shuttle might sound like a nice idea but until capacity in the Oxford area and, crucially, on to Didcot is sorted out, it will be limited to the four a day mentioned in post 14, not forgetting that when/if Oxford-Banbury is wired, then logic says that the Turbo would be replaced by GW semi-fast electric services linking Banbury with Paddington and/or Reading - I really can't see any sense in passing Banbury-Oxford to Chiltern when there is a clear demand for travel from north Oxfordshire through Oxford to the likes of Culham and Didcot. The GW dmu shuttle is in place until 2019 anyway, by which time Network Rail might just have got a handle on its electrification programme and we know what is going on in terms of the Electric Spine, so wires to Banbury may be on the horizon by then.
The Network Rail draft western route strategy also mentions the possibility of wiring part of the Cotswold Line to Hanborough or Charlbury, precisely to assist with removing shunt moves at Oxford/the need for trains to occupy platforms at Oxford station for extended periods.
But in both cases this would be aimed at emu duties, not IEPs, though wires up part of the Cotswold Line would obviously benefit them as well and there remains the possibility that in the next GW franchise bidding, the idea of a - limited - revived Paddington-Oxford-Stratford via Banbury service, aimed at the tourist market and perhaps IEP-worked, could feature again as it did in the aborted franchise process in 2012.
While a Turbo will indeed be allocated for the Cotswold Line halts trains from 2017, nothing has been said about what it might do for the rest of the day, so it could well be used to offer extra off-peak services between Oxford and stations at the eastern end of the line - turnback facilities are available at Charlbury and Moreton-in-Marsh - connecting with some of the extra Oxford-London fasts.