A 3 Car 166 is visibly a tight fit at several stations - not just Crowthore and Gomshall. I suspect to comply with modern standards more than just Crowthorne and Gonshall would have to be lengthened.
First off a 166 has 23m long coaches Vs a 769 which has 20m coaches, therefore the difference in length is 11m.
Next it could well be that ASDO is brought in, as it is cheaper than platform lengthening and could allow for longer train lengths in the future without additional costs, especially on faster services which tend to mostly stop at the stations with longer platforms.
Finally with 3tph due soon (ish), but before the next franchise, it could well be that the case for longer trains becomes bigger of growth takes off (given that the timetable gaps can be 20/40 minutes rather than 30 minutes). As such it could be that the extra revenue could pays for the works.
I don't have precise figures to hand but 319s and other MkIII derived EMUs are several cm narrower than a 16x, and have a greater bodyside taper. As such both the seats and the aisles are narrower. Not by much, but enough to make 3+2 uncomfortable when compared to 3+2 on a 16x, which unlike most applications is actually usable as such.
I don't have accurate figures either, but according to Wikipedia the 165's are 2.81m wide whilst the 319's (and this is unlikely to change) are 2.82m wide.
The narrower bidder profile tends to occur on the 23m long units.
As such the 769's are likely to have broadly the same size seats as currently are used on the 16x's.
So we're now stopping at extra stations to justify the use of trains we don't need? Why not just carry on not stopping at Ash with diesel trains?
I said that it could be possible (Vs couldn't be used) , but the main point was that between Guildford and Ash would (vs. wouldn't be used) be used by at least 50% of all services and most peak services.
I find air con reliability is generally an awful lot better that anecdotal evidence would suggest. This arises from the psychological fallacy that causes people to remember negative experiences, but not notice or recognise positive ones. The 165s are presently being fitted with air cooling like Chilterns fleet, and thus won't have that disadvantage by the time any 769s would be delivered.
In which case it could be that there was an option to fit Aircon to the 769's. Even if it isn't provided it is unlikely to be a deal breaker. Again the extra doors would help as it would allow more of an airflow than the current units.
The better solution for the North Downs would be electrification and use of 387s. I see no desire for a micro fleet of a dozen or so 30 year old hand me downs - especially when the 769s would still not completely oust Turbos from LTV-land with the requirement for a 2 car DMU for both Marlow and Greenford branches. Why have 2 micro fleets - one not particularly ideal for the job it's doing - when you can have one that everyone is already well familiar with.
Of course the better option is for electrification (when is it not).
However it could be that with the 769's you could spread the electrification over a period of time and still gain the benefits from it.
The problem with the 387's is that many of the arguments against the 769's also apply to them, other than Aircon.
The two branch lines which can only take two coach units are going to be a problem whatever happens and so are likely to require a micro fleet for a significant period of time. As such, although something that does need to be considered it shouldn't be a reason not to do something that otherwise is reasonable.
With the two branch lines you just have fitters who are based at Bristol and who are posted to Reading as needed, it's not like GWR don't have any other units of that class. As such, although there could be some extra costs it's unlikely that the costs would be as much as if you had 195's (or some other unit type that GWR don't have) on the branches.