Keep in mind that Special Abilities only count toward an "effective bonus," which is the Enhancement Bonus of the item (+5) plus special ability equivalents (up to +5 with a +5 Enhancement item), and this includes temporary bonues from things like a Paladin's Divine Bond or a Magus' Arcane Pool.
The effective bonus of the item is not the same as the Enhancement Bonus of the item, which is what is used to determine increased AC, or increased Attack and Damage Rolls. Since you have a +2 Impact Greatsword, it has a +2 Enhancement Bonus, and because Courageous increases based off of your Enhancement Bonus divided by 2 (to simulate the half), the increase is +1.
That being said, I would recommend extreme caution, as several players will claim the Courageous Property to not work the way it says it does, so I would ask your GM for his take as to how it works before you choose to get a property on your weapon.
Hope this helps!