Heathrow Express is a vast waste of paths. I sorely hope it ceases to exist when the West Rail Link opens.
The reason I say that is that if you consider these new trains running from Paddington to Temple Meads via Parkway every 30 minutes will have limited stops and the Bath route will always be vastly busier. What if that half-hourly service ran via Heathrow then back out the other side? Yes, journey times would increase but connectivity would be fantastic. Reading, Swindon, Bristol Parkway and Temple Meads having a fast train every 30 minutes to the Airport; with 'one change' journey opportunities to Oxford, Banbury, Leamington, Gloucester, Cheltenham, Newport, Cardiff, Swansea, Weston-super-mare, Bath and such.
The other half-hourly path (aka to make up to 15 minutes) should be 8 car 387s and go to Newbury. It could stop at Slough, Maidenhead, Reading, Theale Parkway (if that is developed) and stations down to Newbury where the wires will obviously stop.
The majority of passengers/businesses from Bristol want to get to London quickly - that is the point of the fast IEPs. Given a WCML-esque journey time and frequency for Bristolians. Only a minority want to go to Heathrow. So you'd be hitting the majority for the benefit of the minority.
The West Rail Link I'd expect to as a minimum have a semi-fast link to/from Reading anyway. So all the places you list as being one change away, would still be one change away.
