I used to do a lot of multi-gigapixel computational photography, and similar size to you photoshop files on a 2010 mini with 8gb of ram - honestly, unless you need more than 16 (and hopefully 32gb soon) of ram you may find the sweet spot for your workflow is a mini with a eGPU, and with all hints that the upcoming (as in "released to manufacture") Retina 5k display IS an eEPU, TB3 eGPUs may well be the "non imac" less over-specced than a Mac Pro sweet spot you're looking for.
I went for a 2009 pro because I needed bulk ram for the panoramic stitching, which eats 20+ GB of ram during the render (and seeing all 16 - soon to be 24 virtual cores pegged to 100% with no change in noise levels has its certain jollies), but it I hadn't needed it at the time, for specific images I needed to make, I probably would have waited to see what happened in the next couple of months.