It is the local stops that eat away capacity. There aren't many freight paths but enough you would need the timetable in the hours they run, to apply through the off peak.
Having a freight path in the middle of the evening peak doesn't look too clever at is would prevent a 4+2 peak timetable if 2 locals were possible.
I daresay it would be better to divert any freight via other routes if it couldn't be accommodated within a 4 fast 2 stopper timetable.