I don't think there's anything to gain at the minute by offing the 158s from there. Capacity isn't needed, and your diminishing the quality of the rolling stock for those undertaking a 3 hour+ journey. Also you confine the Aventras to a pretty linear route that's cut off from any rotation around the network (which ScotRail have thrived off of for the best part of 20+ years), and you create another fleet for Inverness to maintain plus creating a situation where more Inverness 158s have no need to be there in the first place.Far north lines?
And when the 156s have been touted for replacing in the next 3 or 4 years, that's why the GSWR sticks out - routes that need lower dwell times, greater off-peak capacity, and greener more efficient rolling stock. It feels like a pretty direct like for like replacement on the remaining DMU legs out of Central.
Though 10 wouldn't really be enough I'd imagine.