Are they ever doubled up on Birmingham-Stansted?
I guess there may be one or two services which are doubled up but a two-coach Turbostar is what you often end up getting. If it's a three-coach then more often than not there'll be at least one problem in one of the coaches (broken AC, broken interconnecting doors etc.) rendering it uncomfortable!
Also, what about the design of a Networker-derived unit suits a busy airport service?
I don't think the base model it's derived on really makes any difference to its suitability. Both the Stansted Express and Gatwick Express are operated by Electrostars, which have the same weak connection to a Networker as the Turbostar. In fact the Heathrow Express is also soon moving to Electrostars! What it is that I suggested made it suitable is that it has a relatively good balance of seating capacity, legroom, luggage space and door spacing. It's of course never possible to have all factors perfect on a route as varied as those which the trains work (bearing in mind that the diagrams also see the units interwork with Leicester stoppers and sometimes Nottingham stoppers too). But I think Turbostars are generally speaking a very respectable unit.
I use it because it's direct but it's very often a low quality experience of no seats on a shabby train - the Voyagers may need a refurb, but the Turbostars make them look good.
The number of seats would hardly be any better on a Voyager - they have an incredible space inefficiency. But I agree that far more four-coach trains are needed - unfortunately nothing longer than that can be accommodated throughout the entire route (e.g. at Stansted Airport). I feel that the Voyagers and Turbostars are in a similar sort of disrepair so both need working on really!
As for journey times, remember this is a route which sits varying lengths of time at Leicester, Peterborough and Cambridge - I guess to let London services have priority, as usual.
Not really - it's simply that we are talking about an hourly train which crosses a lot of flat junction at a lot of busy lines. The only stations where it has grade separation is Nuneaton and Peterborough. It's inevitable that the routes it crosses will base their timetable primarily around their most important routes, which means that yes, London is prioritised. I don't resent that though - and even with reduced dwells of a couple of minutes it's not going to drastically alter which journeys include the service as the fastest option.