my advice is to try it out in the store. see if the store will install photoshop on it.
i have photoshop (older version, from creative suite 5) on my SP2 with 8 GB RAM and 256 GB storage and Photoshop runs wonderfully. The only issues is my version of Photoshop doesn't support things like pinch to zoom. but the biggest problem is the program doesn't scale at all. So everything is tiny on the screen. and since there's no touch pinch to zoom, the zoom tool will be used a lot more than you may be accustomed to. I never use the Zoom tool so much on my 23" monitor, my 27", or my 20". But on the 10.6" screen, I'm zooming in and out a lot. But the more egregious is how small the tools are in the toolbar. Practically microscopic due to no scaling support.
I read it's a lot better in newer versions of Photoshop but Photoshop is so expensive, I can't justify upgrading when my version is perfectly good otherwise.
I've used Photoshop on 4 GB machines and it's okay. Since the Surface Pro machines are all SSD, using SSD as a scratch disk is not so bad. But yeah, the 128 GB storage is really weak if you plan on using this machine for a long time and to do a lot of work.
it's more about the RAM and storage than the CPU with Photoshop. Not that you don't need a fast CPU but the i5 is plenty good. It's just that when you run low of RAM in Photoshop, it goes to the scratch disks.