Couldn't agree more - there is nothing more frustrating than being on an 125mph HST heading towards Paddington only for the brakes to come on at West Drayton as a 100mph max HEX unit gets priority. If HEX didn't exist today and someone applied to run 100mph max units every 15 mins on the 125mph fast lines into and out of Paddington they'd be laughed out of court.
Function of the access rights being agreed/granted in the mid-90s when there was genuine capacity, and little expectation of growth that soon followed post-privatisation.
In my experience of having observed and travelled on Heathrow Express services they are not carting fresh air all day long. Given that the Great Western Mainline is already planned to have new services added with no real noticeable detriment to HEx timetables (eg. 4tph to Bristol, Crossrail Local services) then is it already not the case that there is available capacity for other services, but no more are further justified?
My recollection is that the high peak Up Main line service into Paddington today (19 trains per hour in the busiest hour 0745-0844) doesn't materially change with IEP.
However the use of those paths is changing, with IEPs having higher capacity than HSTs, 8-car 387s replacing 5-car Turbos, etc, giving a medium term capacity boost. So HEx is fine for the time being.
However, various documents (e.g. Western Route Study) says that more Main Line capacity inwards from Reading is needed longer-term. Whether that's 24tph enabled through enhanced signalling capability or re-allocating the current HEx paths remains to be seen...
Alternatively, if you were looking at railways as a pure business when it comes to allocating capacity, the current commercial return on HEx paths (in terms of revenue generated per passenger mile) must give alot of Season Ticket-filled GWR services that have lost most of their passengers west of Swindon a run for their money...