The difference is VERY BIG.
It has mostly to do with two aspects: playing styles and tatics. For example: Schurrle is marked as a "Hole Player". If you check
here you'll see Hole Players run towards the oppopsition goal when attacking. Not a target man, not a midfielder, great as a winger. I play Schurrle as LWF and it is a blast. Best scorer in the League, 15 games in.
If you have a Classic No 10 and play it on the sides (kinda like Silva, Nasri, Quintero, Óliver Torres, etc), they will play
badly. They don't know how to behave. They are supposed to go in the middle, behind the strikers, and wait. Don't expect them to defend much either.
Finally... The blank ones. These are least affected if played in a position that does not suit their style. They are more affected by...
Tatics and tactical awareness. This is perhaps even more important. Even in PES2013 this was the difference between losing or giving hell to the opposition. Example: Neymar as a LWF, or as a LMF. As a winger he does what wingers do: runs by the line, and either crosses or goes in (Neymar is more propense to be an Inside Forward) to goal. If he is played as a LMF... he won't go until the goal line. He won't go by the side line. He will play more like a... midfielder. Going around the middle, a little more to the left, but hardly to the point of seeing him running behind the opposition defenders.
Another example, this time with both: I was in need of a RWF (I play in a WM formation. Ahhhh, the memories....

), bought Harriott (go see this guy... speechless), he was shite by that time, bought Adama Traoré, another bad call, bought regen Salpingidis, made a few games. And then I found Fischer. I had already bought Andersen back when I still played in 4-2-3-1 for the AMF role, and back then he was already good. Now he was a god. So I bought him. He is marked as SS, but I played him RWF nonetheless. Complete rubbish. Even seemed like he didn't want to run. Long story short, 10 games later I sold Quintero, and instead of buying another AMF I adapated the tatic slighty to use Fischer as SS. It did wonders. In half the games he scored FIVE times the goals, and assisted almost TEN times more.
In summary: don't buy players because they're good, buy players that suit your needs/tatics/style of play.