The 170s are slow to accelerate on the Harrogate line, but it appears to make no difference to the end arrival time compared to pacers and 150s. Perhaps the doors at thirds, and the wide, straight gangway just helps with better dwell times.
One aspect where the 170 absolutely makes a difference is with capacity in the peaks. It gobbles the crowds at Horsforth, with room for more at Headingley and Burley Park, and it does so indivisibly. 3-car 158s seem slower to load, while 2-car 158s are even slower and leave people behind.
If the 170s have to leave, they're going to have to go somewhere else that is busy. They can only realistically be replaced, from a capacity perspective, by a 3-car 158 or a pair of 150 units.
I wonder what stock will be most suitable when an airport station gets added?
There have been fewer on this line in recent weeks with more Pacers and 150s than 170s.
I can only speak for weekday peak hours, but recently 150s and Pacers have remained on the same diagrams allocated to them since December - there might be more units, because they seem to short-form less frequently now.
In the last month, the 170 diagrams have indeed reduced a little, with perhaps 10% being replaced by 158 (rather than 150/14X) split 50:50 between 3-car and unsuitable 2-car units.
From my observations while commuting, March produced one of the best months for capacity on the line, especially in the AM peak, where 92% of traction was no smaller than the 3-car turbostars (81% being 3-car 170/158, 11% being 4-car 150).
I was quite liking the 170s for my commute from Selby to Leeds.