Agreed.
As others have posted, any significant acceleration of services between Waterloo and Exeter seems unlikely due to the constraints imposed by the route and infrastructure.
However "proper mainline trains" does not just mean faster.
A proper main line train is gangwayed along the whole train, has a buffet, and possibly a restaurant, has largely facing seats across decent size tables, ample stowage for cycles, surfboards and bulky luggage, and first class at one end.
A cobbled together collection of DMUs simply does not feel like a proper mainline or inter city train.
Not certain that an 800 or derivative thereof is much better than the present DMUs.