this is sad, you cant compare these players for many reasons, the biggest one being that a player in Nigeria for example could look World Class in Nigeria and look crap in EPL, and a player from the same 'example' could also be better, you dont know how a player will re-act to that step up, sometimes nothing happens, sometimes they step up and become a better player because of the challenge.
Example: Frank Lampard...
3 years ago, most people wouldnt rate him in the top 50 of midfields, thats why when Abramovich came there was so much spectulation he was going to be sold, now he is arguable there best player, he stepped up to the challenge.
Example: Paul Scholes...
Out of all of the Class of '92, Scholes (besides Becks) looked the most unlikely to make it in the first team, but he stepped up and became one of England's greatest ever players (IMHO)