From memory Michael Cooper has played just one competitive game against a Premier League team and a handful against Championship teams. Given that the pace, power, movement and shot speed of players at higher levels is significantly better than those in League 1 I think it’s pretty much impossible to accurately compare his ability with keepers already playing in the leagues above League 1.
I think experts in the game ‘just know’ through various ways when a keeper has the ability or just potential to play at a higher level. E.g. City (pre-Abu Dhabi money I recall) took Joe Hart from Shrewsbury pretty confident that he’d be a top keeper for a decade or so.
Even without being an expert it’s fairly obvious that Cooper has all the core attributes required, that can then be moulded into a top class GK at a top club. Which is why I have a bit of a hunch a club like City might take a punt.