165s and 166s would only get moved onto other services if electrification of at least the Thames Valley lines were electrified and caused a big displacement of Turbos: Releasing five or six 166s from introducing five Adelantes on the Thames Valley services won't cause a major shake up.
So, on a bit of a tangent, if there ever is Thames Valley electrification, then it seems perfectly reasonable that, with the relevant infrastructure improvements, the 165s and 166s could head over to Cardiff to Portsmouth/Paignton and Great Malvern/Cheltenham to places south services: There's 58 165s and 166s operated by FGW, and 41 of them are three carriage trains: With that sort of size of fleet to play with you could very easily operate pairs of two carriage units or a two carriage and three carriage combination with the current frequency and timetable, and handily rid the West Country and Bristol area of Pacers through cascade of 150s (Of course, the 142s are going sooner than that anyway).
The Thames Valley, and through to Worcester via the Cotswolds, is a very suitable route for the 180s: It's a shame that they were so troublesome in the first place.