I think their best utilisation would be for high frequency services along routes with line speeds of 105mph or higher; such as Edinburgh - Newcastle, Preston - Birmingham, and so on
Both of which are electrified, and running diesel under wires is never desirable.
The Central belt-aberdeen argument would be that you could then have a proper FC area, with 'intercity door spacing' something which seems to bother the Aberdonians a lot. Also, the Turbostars could then do the stopping services, and the voyagers run fast to reduce journey times from the central belt. To reduce pathing issues you could run a double set non stop aberdeen-Dundee, then split one for Edinburgh and one for Glasgow - maybe even into Glasgow Central for ease of interchange.
Better than 158s(From Commuters point of view) but worse than the 33s they use to have on them