I know I'm late but I'll still leave my input. In tests the 2.6 is about 6% faster which you will only see in applications like Photoshop or heavy video editing softwares. During normal tests you will not see a difference. If you want to "future proof" you laptop then get 16gb ram. That will increase your speeds in many ways. If your a heavy gamer then get the high end 15" model as it has a better gpu, (actually the early 2013 model has a better gpu then the baseline new one). So, as in my case, I'm going to be using this for years, I got the high end model. I wasn't going to get the 2.6gpu processor but my education discount saved $200 and was worth it to me. In the end you really don't need the 2.6ghz processor unless you are going be using extreme programs.
I had asked a similar question, but didn't get a response that I was hoping for. I still haven't found a good answer.
In the end, I bought the $2600 model and am thinking it will be "future proof."
2.3ghz quad core isn't bad, but the major selling points were the discrete graphics. 16gb ram should last a while. 512gb ssd isn't bad, but ideally a 1tb ssd would have been great, but i'm not made of money.