As well as solving the capacity problem on the route, I'm sure the improved rolling stock would in time make a big difference to passenger numbers.
... resulting in a capacity issue once again?
Given the current sets are 3 car units, 5 coach sets should be OK for a while. Ideally the services should be limited in stops, with the other stations served by shorter services (e.g. Westbury to Bristol, covering the Trowbridge and Bradford stops). That would also create more capacity on the
HST▸ 's and reduce journey times, making the services even more attractive for longer distance traffic.
Although from what paul7755 says, it isn't going to happen so maybe this discussion is academic. (Or maybe not, as the principle is the same even if the service is run by 5 car turbos.)
I probably just hanker after the service I used to take to get from Cardiff to Soton when at Uni... only 4 intermediate stops.