Should be all right
I m on a late 2008 macbook pro with a crucial ssd and 8gb ram. Photoshop runs like a breeze up to 8000 x 4000 px and about 500mb per file. (In some cases i went up to 2gb per file and wondered why background saving took so long without any other performance issues.)
Actually ps runs faster than on the non ssd mac pros at work.
Ps CC 2014 does help a lot, the improved graphics tablet engine feels awesome and background saving is great for large files. Just make sure you get the max ram, that is usually the bottleneck for photoshop.
I don t sketch a lot on the way too small macbook screen, but on an external monitor. Therefore the retina screen is kind of obsolete for me and the small ssd would be a bummer. Especially as Photoshop eats easily 50gb scratch disk space.
A 8gb ram 256gb ssd Air might be a better choice as its way cheaper? Not sure though...
Hence I am personally waiting for the new macbook line in 2015 for the more fashionable looks. 🙂