Depends on the rest of your team, if you have a world class squad for instance there will be no player in the game that will be 6 - 8 stars. My Sunderland squad in 2012 for instance, they are mostly world class and my star players who are worth £25 million on paper (but I'd never sell them for less than £75 million) are 5 stars, and they are world players of the year and get golden boots and stuff.
Although if you had a League 2 squad and brought in a rubbish Premiership player, chances are he'll be 6-8 star as he'll be far better than the rest, but rated against world class players he'll probably have 1-2 stars.
messi was 5 star at barca, but now he's 4 at my club, how does that work?
and what do yo mean 6 - 8 star? i thought 5 was the highest?