What's a 'convincing win'? A win's a win, right? One more vote than Paulbot or Santorum. And right now Mittens is on top.
I'd say anything over 5 points in Iowa is a solid win. In politics, you also have to beat the spread as well as your opponent. A 1% win is a tie, or in many cases a loss when it's looked at by supporters. If Romney had beat his expectations (5%+), he would be sitting in a very nice position. Now he has to shore up his game in Florida quite a bit.