GWR would still have 30 five-car IETs if you did what I suggested - I think that would be plenty of versatility. In all, they would have:
- 30x 5-car bi-modes (8 class 800s and 22 class 802s)
- 35x 9-car bi-modes (21 class 800s and 14 class 802s as now)
- 14x 9-car EMUs (class 807)
New sets for XC would presumably mean new diesel engines and GWR running bi-modes under the wires all day instead of getting some EMUs. I suppose you could just take the diesel engines out of the GWR bi-modes and use them in the new XC units. However, running 5-car 800s in pairs is inefficient (10 coaches of weight but seating capacity is basically equal to a 9-car set, and you can't move between the two sets while the train is moving), and I think GWR has too many driving cars in it's current IET fleet so at the very least I would like to see the new XC fleet using ex-GWR driving cars.
Aren't all vehicles powered on both 175s and 180s? In which case I assume the drivetrain and/or the diesel engine itself are different on the 125mph class 180 compared to the 100mph class 175 which I suspect would give some issues if you did try to combine them.