In photoshop there won't be any notable difference. In Maya, rendering times will be about 15% quicker on the 2.6GHz model. No idea what unity is. For Final Cut X, video encoding time will also be quicker on the 2.6GHz model.
Parallels relies more on RAM rather than on CPU processing power so there won't be any notable difference for that.
Maybe could you list down the specs of your previous iMac? That way we can tell you how the new MBP compares up to it. Generally, laptop processors today are more powerful than desktop processors 2 years ago.