Did you factor in however that most passengers would prefer to walk through Piccadilly to get to the City Centre, Piccadilly is already on the edge of walking distance and Mayfield ground level would have very poor links. Even for getting to the nearby Manchester University the roads on a viaduct.
Yes, most foot traffic will be using the new link into the Piccadilly concourse that will direct pax flow past the shopping units crescent.
You'd be surprised how many pax make use of Fairfeild Street though...
But yes, congestion at the Piccadilly entrances is a problem today, but I can't see any answer that integrates anything into the current station that doesn't result in more foot traffic coming in through the gateway house entrance.
Re-modelling Fairfeild Street in a major way and re-directing the MetroShuttle buses down that way would help significantly, (could make use of the current Travis St Triangle to turn round) and lest not forget that taxis already pick up down that end, and a significant number of 'thick' passengers that don't know Manchester head straight for the Taxi rank.
Honestly though, short of re-opening another station in Central Manchester there isn't any other way to relieve foot traffic congestion at the stations as it it, other than when Piccadilly does inevitably expand, providing as many routes and exits as is sensible. Such as direct access from 13-16 to Fairfeild St and an additional link way from Costa to the remote platforms, with the existing one demoted to linking to other platforms and entry or exit only from 13/14, I'm favouring either.
The only realistic ways to take pax away from Piccadilly is to run more through to Oxford Road that is in a worse state for Congestion than Piccadilly already is. Or stop this obsession with sending everything through that way and blow £300mil odd re-opening another major station in Manchester. Reducing the load on Oxford Road, and Piccadilly, or at least preventing it rising any more.
The 'cheap' solution being re-opening Exchange as that helps out the Windsor and Chat Moss lines a lot, but still leaves the CLC knackered for much more growth, although Exchange if done properly with a good and expensive junction at Ordsall Lane would mean that Windsor Br. traffic could be reduced (slightly) at least until Leigh re-opens and never likely to grow beyond 14tph through Crescent and 10tph through Victoria, easily handled by Platforms 5 and 6 (current) and some turnback sidings or pushing services through to Rochdale and Stalybridge.
Grade seperation at Ordstall Lane is needed for any future growth, as is major track additions between there, Deal Street and Exchange. With a predicted service level (not committee, predicted sensible levels...) of something like...
4tph Oxford Road and Airport
2tph Wigan NW via Eccles
2tph Liverpool Lime Street via Eccles Slow
2tph Liverpool Lime Street via Eccles Fast (TPE)
1tph Chester Semi Fast (Possibly 2tph with 1tph terminating at Wigan NW)
One can see a very congested set of platforms at Victoria even with reversing sidings and a very conservative timetable. The addition of Exchange platforms with a grade separated link onto the Chat Moss via Ordsall Lane and four track between there and Deal Street (2 tracks for Cord and current layout for Windsor Link diversions, 2 tracks for the Chat Moss via Viaduct).
This will be able to take all but the Cord and Liverpool Fast services, dropping through platforms for the busiest services down to 6tph, sharing could be put in place with 5/6 when busier but shouldn't be needed (Windsor Bridge is likely to remain at 5 + 2tph for a while, possibly drop to 4 + 2 for DMU saving, cut Wigan - Manchester via Bolton at Bolton, pax travel via Eccles instead). So you'd be looking at 6 or 7tph per pair of through platforms, if one includes freight, that's rather sensible. Terminating platforms themselves could independently take anyting up to the limit of the Chat Moss Route (I'm theorising with some passing loops running the length between Eccles and the M60) that you could jam 12tph down there.
So the Chat Moss will be seeing... (Between Manchester and near Newton)
1tph Manchester Piccadilly (Airport) - Scotland
1tph Manchester Piccadilly (Airport) - Llandudno
2tph Manchester Victoria - Liverpool L St Fast
2tph Manchester Victoria - Liverpool L St Slow
2tph Manchester Victoria - Wigan NW Slow (You say slow, there's only two intermediate stations!)
2tph Manchester Victoria - Warrington BQ (1tph to Chester)
Since Liverpool - Airport is being sent via the CLC as an extension of a Diesel service at the moment, thats the Chat Moss sorted, if Leigh Parkway gets built, the slows to Liverpool L St and Wigan NW would call there giving 4tph of a 4 car EMU. Possibly 8 car in the peaks.
Services via Bolton and Atherton to Wigan would become quieter as people would start using the new 30min services into Manchester from NW, so one can afford to save a DMU or two by cutting the Manc - Bolton - Wigan services to a Bolton - Wigan shuttle, replacing this with additional EMUs between Bolton and Manchester. Airport - Southport diverted via Atherton.
With many EMUs running about the place, if need be and growth demanding? Insane length EMUs can be run from Bolton as a crowd buster, calling at Piccadilly or Victoria only, as nothing else can take 8 or 12 cars in Manc. But enough pax should have been removed to allow 6 car 323s to be enough in the peak and 3 car 323s off peak.
Thinking something ITR of through Bolton into Manc...
2tph Blackpool North (EMU)
2tph Preston (EMU)
1/2tph Blackburn & Clitheroe, putting the loop back in, push it to 2tph?
1tp2h Lakes / Barrow (DMU)
1/2tph Bolton or Horwich Parkway (All stations to Victoria) (EMU)
Other services being:
2tph Wigan NW/Wallgate via Westhaughton.
Noticed that I'm trying to cut DMU services at all?