added 2 sticks of ram....

have a mid 2011 27" IMAC with 2 sticks of 2gb ram. Added 2 sticks of 8gb (dropped the 2 sticks of 2gb down to the secondary slots). Big difference in speed....BIG difference. Will wait and see how it helps in my workflow. Have been told this particular IMAC can handle 32 gb. In any event I may save up to replace the 2 sticks of 2gb for something else. Kinda like life, once you get a taste of it, you want more.

By the way, I ordered it from crucial themselves....got a nice deal on a usb (8gb) drive but I should of ordered it from B & H. Cheaper, found out later that I could of saved at least $20 (no tax for one thing).

You won't really see any advantage going beyond 8GB right now, at least not for photo editing or general usage. I fell into that trap with my Mac Mini too and bought 16gb for it, but if you look at a system monitor you can see exactly how much you're actually utilizing. LR and photoshop together generally only use between 4-6GB of RAM, and that's with the internet browser open too for me.

Also, I would remove those extra 2GB sticks. Unless they're the exact same specs as your new stuff (unlikely), they're going to cause conflicts and random slow downs, they're not really doing anything for you in there anyway, so I would just take them out.