I used to work in Amesbury very near this site and I can tell you this completely unnecessary.
Sure fix the potholes but 2bn for 2 miles of duel carriage way that ultimately won’t speed up journeys between London and the shitholes on the A303 (eg. Salisbury) just aren’t worth it.
It would move the queues to the next bottleneck. "Just one more lane bro" planning never works.
Rail between London and Cornwall needs improving instead, as does local transport around Salisbury (active travel, public transport) as a large portion of the queuing vehicles are local drivers avoiding congestion in the town.
I mean, it's a bottleneck heading back towards London too, between the barrow roundabout and Stonehenge is often a mess in both directions. Mostly due to people looking at Stonehenge instead of the road.
It's less about improving the trains and more about making them cheaper! (Not that I would complain about some new lines either!)