If Stranraer gets a more frequent service to Ayr where passengers can change to a comfortable air conditioned electric train with one leaving every 15 mins it doesn't seem too bad.
The increase in frequency between Stranraer and Girvan would certainly encourage me to use the train more often when I'm visiting Portpatrick. The current timings means I tend to take the bus instead.
I'd agree that focussing on Stranraer-Glasgow direct isn't necessarily the best way to go with this line. Although direct services do have distinct advantages, particularly to those travelling with luggage, a transfer to a Northbound service at Ayr would be easy.
But running trains preferentially to Kilmarnock will not only maintain rail services to Stranraer, but provide a valuable link between Ayr and Kilmarnock (which has only a handful of daily trains). The frequency of Glasgow-Ayr services mean that there would be minimal, if any, significant increase in journey times. I'd love to see hourly Kilmarnock-Girvan trains (I think there would be demand), with extensions to Stranraer as needed. Continuing service from Kilmarnock to Glasgow/Carlisle would be possible as well, although I suspect that the market for through traffic would be minimal on the whole.
The only concern I'd have would be Southbound: in the event of a delay on the Glasgow-Ayr portion, would it be possible for the Southbound service from Ayr to be held?
I'd argue the best way would be:
1) Connect Stranraer services to fast Glasgow services. Without exception. (If you want to go to local stations, the slow one won't be far behind).
2) Ensure that a fast Glasgow service arrives on P2 for a cross-platform transfer to P3.
3) Vice Versa in the opposite direction
4) Hold Southbound trains to Stranraer as long as possible to allow the booked transfer. You'd need to work out the maximum time based on the single track line to minimise ongoing disruption.
If it's done right, stations South of Ayr have the potential to improve their overall service for the sake of a transfer.