15/16 Plans include an extra terminating bay at Oxford Road, and making Platform 1 accessable so more trains can make use of it. Not a huge amount more paths are to be added, this is very much a reliability move.
Unfortunately, the ship has sailed on everything to the airport with the construction of the Ordsall Chord!
What 15/16 will do, is allow current services to run more reliably, have an extra 1-2 trains per hour stop at Oxford Road, which is close as you can get to some westward terminating bays at Piccadilly!
The incredible amount of crowds you see on those platforms are a massive sign they are overcapacity, plus the shouty people and red lines. Similar situation with Oxford Road.
Wether the platforms are the bottleneck to train movements or not, they are undeniably close to capacity passenger wise. Less frequent 6-8 car trains will still face issues of not having enough space for people to embark/disembark. Passenger numbers can only be expected to grow, as Manchester and the surrounding North-West become increasingly popular places to live, more move into city centres and people ditch their cars.
The stairs, lifts and escalators at 13/14 are rammed for almost the whole day, with rush hour being pretty agregious.
Dwell times are also high, as the trains empty a lot of passengers passengers and then completely fill to standing room only.
Flyovers and grade seperate junctions may help trains move about more, sure, but when it comes to dealing with the actual passenger volumes, two more platforms seems very reasonable.