Didnt West Coast use shortened HST's to stick to voyager paths (4/5 coaches?) Maybe if we added another power car to a HST set, we could get better acceleration.
That would release a fair few MK3's into service, and the other car from each set could replace the MK4 DVT on IC225 sets, to allow better acceleration, and in turn those DVT's used with the spare HST coaching stock with newbuild Traxx Locos or similar
Virgin Crosscountry operated 2+5 HST formations to allow them to keep to time with Voyager schedules, I'm not sure whether the West Coast HSTs along the North Wales Coast were similarly shortened.
Could you clarify what you are proposing here? How/where would you add a third power car to an HST set? And if you were to do that, how would there be any spares left over to work with mark 4 rakes? Plus, that then raises the peculiar situation of having diesels running under the wires as part of an electric formation.
Although I do think that replacing the DVT of a mark 4 set with an HST power car would be a "quick fix" to do away with the requirement for HSTs on East Coast, so that mark 4s could operate the off wires services. At the time of specifying the production HST power cars, a 3,000hp engine that would fit into the bodyshell was offered (and of course the East Coast class 254 sets were originally meant to be uprated at 2,500hp), but it was turned down on the grounds of increased axle weights and fuel consumption when a 2,250hp power unit was all that would be required to meet the timings envisaged. If the HST power cars intended to be used with mark 4 rakes could be retro-fitted with such a power unit, that would ensure they had enough "grunt" to maintain timings off the wires.
Of course, it wouldn't be possible at the moment as there are insufficient mark 4 rakes to go round as it is, so it would require a new build of trains to replace the 225s on the electric services.