No, it's not 5%. You'll probably lose some possession if you have several players out of position but I'm not sure of the number, it's just speculation until Nordeus say what happens. I do know for a fact that if you play players out of position the less effective they become.
I did play a game where there had a similar feature. I managed to get a hold of the SQL code for some of the values it stored and the number of squares away from the normal position of the player affected their performance... So that was based on a percentage, a striker playing as an AM would be 20% less effective and a defender playing as a striker would be 90% less effective. I'm not saying that it IS the case in this game but it could well be a similar thing.