And again how is 2 more cores "much" faster then the 13"? If your getting a base or mid tier 15" all your getting is 2 more cores, a slightly faster iGPU and a bigger screen.
It obviously depends on the work load. I thought the model OP was looking at had a dedicated graphics card though, even then. If you so a lot of batch processing, and need some of the more intensive tools in Photoshop like photo-align (mainly used for panoramas, multi-exposure shots, focus stacking), content-aware tools and some of the more intensive filters like smart blur the processing performance will be very noticeable. But you're right, most of the time when doing photo retouching, you're not waiting on the computer to keep up with your work.
RAM could be the clincher though, if you've got large files open in InDesign, Photoshop and Illustrator at the same time (let's a throw in a web browser for good measures) all is well until you start running out of ram and life just becomes miserable. While he may not need it everyday, having more ram gives you much more flexibility in your workflow.
But you make a good point, while it is a much faster machine, the difference might be pretty imperceptible for photo retouching. I personally feel the difference for video editing and 3D rendering where having twice as many cores make some of long wait times half as long.