Considering the 80x have better acceleration than the Pendolinos, and can't take advantage of tilt acceleration where it's best needed up in Scotland anyway, it was decided to make this stopping service Evero operated.
The 807 order was larger than it needed to be to cover Voyagers to allow for a second Liverpool service each hour, and this is why they've started to appear on Liverpools in place of 9-Pendolinos.
Point one, the Class 807's slightly better acceleration is only worth 20 seconds from a station start if both trains are running at the same top speed. But where an Evero is running at only 110mph , this advantage is short lived and the Pendolino is level after 11 miles and now heading into the distance with a 15mph superior top speed.
In reality this translates to an 807 being around a minute slower between Euston and Watford Jn, Watford Jn and Milton Keynes and Milton Keynes to Rugby sections. Of course we have to remember that the 807s have to reduce speed in several locations where the Pendolino's are still allowed the full 125mph or a slightly lesser 120mph.
That's why the Euston to Liverpool services are timed slower than they would be if expected to be served by 390's.
I note the latest timetable sees a 2 min earlier start for 807 Liverpool services - presumably to make up for the fact they cannot run at the proposed MU limits. But that is currently offset by the ongoing signalling issue near Euston which is creating a 2 to 3 minutes delay to all services on the fast lines through Primrose Hill tunnel.
Point two, Avanti publicly said it ordered the 805/807 fleet to reduce running on diesel and eliminate diesel running under the wires - whilst increasing some service frequencies - notably the second service to Liverpool per hour.