No performance issue at all. If you're running the cards at stock there is no need to consider synced or not. That only really come into play when you are trying to get the most out of each card in which case you would have them un synced. Because if you put a +150 on the clocks one card will crash before the other, so you might need +130 on one and +150 on the other. This will only really coming to play at the absolute limit of the cards.