I am not going to give you a long tech speech about how many watts on a 12v rail that card requires. The Rosewill RP is in the tier 3 chart for PSU's It is stating that this model is Temp sensitive and it is on the line between good and bad.
With that being said I would think when your PSU gets to a certain temp then it starts to cut down on power to protect its self. This would be my opinion as to why you think you are bottlenecking. Since the PSU didn't come with the proper amount of molex's then maybe you upgrade the PSU.
Now the 560 requires a min of a 500w PSU so here is a link to a 500w and a link to a 750w. You can check out more on your own but try to stay with the Antec, Seasonic, XFX, and the Corsair brands. I hope this is the problem. Good luck to yahttp://www.newegg.com/Product/Product.aspx?Item=N82E16817371035http://www.newegg.com/Product/Product.aspx?Item=N82E16817371049