On the specific issue of whether 323s are essential to the interworking of services between Piccadilly and Stoke I make the following comments:
1. I've always been surprised that door release remains with the guard. Apart from anything else this eats up paths that cost a fortune to supply. What's the point of spending a fortune to resignal/upgrade the line only for capacity to be limited by the failure to allow drivers to do this task? In my experience (my parents lived in Poynton for many years) standing by doors awaiting release, whilst guards attend to ticketing issues, is very common.
2. With two locals per hour at least one stop (Adlington) is expected to be cut from the through service to Stoke.
3. With four tracks between Slade Lane Junction and Adswood (assuming the western pair north of Slade Lane will basically be for Airport-Oxford Road and beyond services) there is scope for Macclesfield line locals to run slow line, call at Levenshulme and Heaton Chapel, be overtaken and re-enter the fray of the main line immediately behind a fast train thus maximising the time available to get to Stoke. Similar arrangements could apply northbound. At present the standard timing is for the locals to leave Stockport 12 minutes behind the preceding up fast.
I conclude that, with attention to some or all of these points, trains with lesser performance ought to be practicable for service on this line.
Having said all this, the sense that the North is last at the eating bowl, and that what little it has is fair game for everybody else, is growing. Perhaps greater clarity of, and commitment to, a bright new future wouldn't go amiss?