No to the High Speed line, and a tunnelled line paralleling the overcrowded section is preferable to the disruption of six-tracking. It just needs this development to justify and fund it:
A super-dense (and private car-free) new town “West Windsor” of up to 200,000 people straddling Drift Road, intended to part-solve the housing crises of unaffordability and insufficient housebuilding.
Supported by a new road from M4 J8/9, plus a 10tph Elizabeth Line branch:
A portal on the Acton-Northolt line immediately west of Old Oak Common (OOC) leads to HeathrowT5b and WestWindsor underground stations. Line surfaces at Suttons Business Park / Gasworks with new surface tracks parallel to the North Downs Line into Reading platforms 7&8.
Current HeathrowT5 Elizabeth Line services plus most OOC terminators take this new route, now terminating at Oxford (2tph service with 2 stops before OOC), DidcotParkway (2tph 5 stops), Basingstoke (6tp2h 5 stops), and Newbury (3tph 6 stops).
Mostly running Semifast, they replace these station's 8tph of local Stopping and Fast services which currently terminate at Reading or Paddington. Only the Oxford-DidcotParkway Stopping service is not absorbed and extended.
Overcrowding would be eliminated and 5tph of additional long-distance trains are now possible into Paddington. Some relief for Chiltern and SouthWestern mainlines too.
Crucially, half or possibly all the £10Bn cost can be privately funded + no need for the inferior £0.9bn Western Approach for Heathrow:
-£4.5Bn from Developers: £50k value uplift from Elizabeth Line on 90,000 new West Windsor homes;
-£1.0Bn from Heathrow: gets faster and more frequent services, but more importantly an ample supply of cheaper staff a 10-minute train commute away;
-£4.5Bn+ more from Developers possible: if the government buys the currently normal-value land for West Windsor, and sells it for with-planning-permission prices.
Service levels:
OOC 10tph*. Reading 10tph. WestWindsor 6tph*. HeathrowT5b 5tph*. Basingstoke, Newbury, Thatcham, Theale, ReadingWest 3tph. Oxford, Cholsey, Goring&Streatley, Pangbourne, Tilehurst, 2tph. Chineham (new station), Bramley, Mortimer, ReadingParkway 3tp2h. NewburyRacecourse, Midgham, Aldermaston 1tph.
*Add 2-4 extra tph terminating at WestWindsor as the town grows and nears completion.