From my experience, even from level 1 the com does fairly well at keeping possession of the ball, and is still capable of scoring 35yard screamers, if you give them the space that is. however, when it comes to defending, there is a very noticeable difference on the lower levels. On level one you can pretty much walk your way into the box, on level 5-6 you'd be lucky to take two steps without being closed down left right and centre. As for goalkeepers, i think that they are the same whether its level 1 or level 6 when it comes to actual shot-stopping, the only difference i would say is that they are less likely to come steaming off their line to catch the ball or to clear a slightly over-hit through ball by the opposition etc