You won't always see speed gains, but sometimes you will. Depending on the intensity of the PS project, for instance.
While I was waiting for my ram to come for my latest iMac, I was at the stock 4GB. While, ok, I could see certain things hanging when I had a lot going on. I wouldn't dare try and push it, either.
Now that I have my 16, I see no drop in performance all while listening to music, encoding a video, have 18 browser tabs open, transferring large files from external AND encoding a photoshop file. All with random apps "open" but not being used.
Not faster, but *stronger*.