The 197s certainly have the ability to reduce times over the current fleet. Even on the most economic transmission setting they'll comfortably out accelerate anything in the existing fleet, and that's before you take into account the much improved dwell times that the door layout will provide.
With the 67s and MKIVs it gets more complicated. Their acceleration at low speed is relatively poor, but once they get into their stride they take off. They'll also out climb any DMU, and the Marches line is notoriously hilly. The door layout is not very good for short dwell times however. Also as it stands there are sections of the route where they will have a lower top speed (ironically there's also one section where they'll have a higher top speed, but it's not likely to make as much of an impact on timings).
There may or may not be space to cut a few minutes here and there from the schedule, and on routes with more frequent stops and most if not all services being worked by 197s hopefully we'll see that. With the Manchester Cardiff services likely to alternate between 67/MKIV and 197s each hour and the need to make a timetable both fleets can match, I think it's more likely that a reduction in stops will make a much bigger impact on journey times.
But like I said, until we see the timetable it's hard to say for definite.