Indeed, the 36 miles between Long Ashton and Cogload would save 4.5 mins, plus anything you might get south of Taunton. A 5 min saving on the hour between Bristol and Exeter is not too shabby, particularly if a similar improvement could be yielded between Bristol and Birmingham. If the govt wants to get people out of their cars and also generate modal shift on the longer distance cross country rail routes it needs to look at all the areas where incremental improvements can be achieved. In isolation each one doesn't appear much, but added together start to make a difference. Likewise, "levelling up" should mean trying to improve connectivity between the regions, not purely a London-centric approach of improving the regions' transport links to the Capital.
And I agree that speeds west of Exeter are a big issue, but that is a separate issue, and off topic for a thread about 125mph capability on XC services, as any increases, whilst valuable, would still be well short of 125.