There are a lot of platforms designed to fit 6*23m in Scotland, so I think Edinburgh-Falkirk High-Glasgow is the only place where 8 car running is possible. Stirling will presumably need to be 6 car in peaks so easier to operate with 385/0s and reduce to 3 car off peak. Though a bit of creative diagramming might allow 6 car to Stirling and 4 car via Grahamston for peaks, then 4 car to Stirling and 3 car via Grahamston off peak. At least none of the routes are getting a 2 car 158 any more.
Though the Stirling-Edinburgh route, as not served by the Inter7City network, strikes me as the only viable route other than E-G where they could actually sell some first class tickets if platform lengths and diagramming allowed it.