331's really are just what has been needed for a long time.
331s aren't much use where there aren't any wires.
The sluggish 142's / 150's / 156's always struggled to reach line speed in too many places. Even 319's, to a lesser extent.
There won't be the financial justification to purchase more for some time.
There's so much variation between performance of individual units that any traction effort info is meaningless.
The average 150 on its own with an average, non-excessive load (say 30-50 passengers) might reach say 70-75 on the gentle rising gradient from Salford Crescent towards Bolton, assuming clear signals & no running brake test needed. Two 150s in multiple should reach 75, anything less and you know you've potentially got a dud one (or slightly dud pair). This doesn't include faults such as engines reverting to idle, which is usually obvious to an experienced driver.
A poor 150 might manage 60-65 on the same stretch. A decent one will get to 75 no problem and once there run at that speed with only a partial traction power setting.
Even a poor 319 will reach 75mph without breaking a sweat up to Bolton. However no 319 will reach 100mph, they're just not capable, 85-90mph is about all you can expect and that will take forever.
I've travelled many times in 25 year-old 319s on the long climb up to Elstree tunnels on the MML down fasts, they often exceeded 90 , - even 95 wasn't impossible
By comparison a 331, even a 4-car, will reach 100mph on the same stretch very very quickly as soon as the line speed permits it (line speed is 95mph after Agecroft, 100mph 1/2 mile after Clifton). Even without taking full traction power and when heavily loaded a 331 will easily do 100mph on that route. The difference between those units and everything else Northern operate (even 323s) is monstrous.
As I've said above, the 331s need wires. The 769s are intended for use on non-electrified lines like the Southport branch.