Lindespeeds on the Berks and Hants are about as fast now as they are ever going to be. The GWML via Bristol though is suitable for upgrading to higher speeds. If this results in the removal of the time penalty that would result in running West of England services that way then there'd be no reason to use the Berks and Hants for these services and the line could be downgraded to a secondary semi-fast route to Taunton.
It's the same on the roads, the A303 hasn't been upgraded to full dual carriageway even though it's the shorter route to the Westcountry and one reason is that a high quality route exists via Bristol that is just as quick.
That depends where you start from though. From Epsom it is about ½ hour faster to use the A303 and A30 to get to Devon and Cornwall than it is to go via Bristol on the M4.
Similarly, the Berks and Hants would be quicker for people from, particularly, Newbury to reach the West Country - and the number of passengers who do board westbound HSTs at Newbury looks pretty healthy to me everey time I have observed it, most recently last week.
Could the Berks and Hants be sped up? Well, if it was electrified throughout then yes just from the greater performance characteristics of electric over diesel. Maybe only a 10 minute gain between reading and Exeter though?
In terms of linespeed - it would probably be similar in engineering terms to the work done over the past couple of decades to straighten out the bit of the WCML that runs via Weedon but the difference is, of course, that in the latter case they had a two track railway on a former four track formation, so the earthworks and land ownership were already in place. The B&H, on the other hand, was built as a two track formation and therefore does not have this advantage.
So the question of improving speeds on the B&H is probably more hampered by land aquisition and earthworks costs rather than any actual engineering issues.
( Wasn't the Trent Valley section of the WCML straightened out somewhat as well? )