What's the problem? The journey may only take six minutes, but it runs every 20 minutes, and at peak times is well enough loaded to justify that type of set. As already pointed out there isn't anything else available in the area, and drafting in something else would require more sets as a spare would also be needed, but at the moment a spare can be drawn from the wider pool. The only other option would ba a complete recast of the timetable to run the services to and from Paddington, but as there are no off-peak Paddington > Slough terminators any more there's nothing to link it up with, and you'd have to cross the fasts at Slough to get to the slows so that's a total non-starter anyway.
The service and traction are fine as they are.