I don’t know, but I’d be surprised. Alignment will be one reason for the restrictions, but it is so curvy that signal sighting will also be apart. I also suspect (my educated guess) that west of Newton Abbot the track is ’Cat 3’ maintenance, and raising the linespeed by, say, 10mph would tip it into ‘Cat 2’ which doubles the patrolling and inspection frequencies.
In any case, allowing higher cant deficiency at that speed may only give you 5mph, and maybe not even that. I’m now going to get a bit technical, and my memory is rusty, but…
IIRC the cant deficiency on the WCML Pre upgrade is the maximum ‘normal’ level allowed in the track design standard of 110mm (Essentially this means that at linespeed, the track would need super-elevation of an additional 110mm on the outside of the curve for a passenger to not feel any lateral force). Pendolinos can run at up to 300mm cant deficiency at the rail, but this is offset by the tilt, meaning passengers only ‘feel’ the 110mm. But the 190mm of extra cant deficiency at the rail is what won the speed uplift from 100/110 mph to 125mph; ie increasing the cant deficiency by 170% enabled speed to be increased by a maximum of 25%, and less in many circumstances.
‘Exceptional‘ curving rules lift the cant deficiency to 150mm, ie an increase of 40mm, or 36%. I suspect this enables speed to be increased by around 5-10%. That’s an increase of 5-10mph when the linespeed is around 90/100 as on the B&H or MML, but when the linespeed is 60mph you may only get 5mph, if anything.
Perhaps an actual track engineer can comment on the effect on linespeed of exceptional curving rules at 60moh and correct my amateurish explanation.