properly adjusted you probably won't see too much difference.

I definitely have noticed a difference between my Dura-ace cervelo vs my 105 specialized over the past few years...never had any issues with my Dura-ace and shifts just so smooth and quickly but the 105 was always good and not an issue. I don't notice much difference between the Ultegra components I have now on my 2012 Roubaix vs the 105 components I had on my Allez. I do know my Roubaix is (in general) much lighter and tighter and stiffer than my Allez so it feels a lot faster. I assume the weight difference in components helps to contribute to this.

if you can afford it I would step up to 105 just so you don't want to upgrade sooner than you really need to.