I think you'll see the biggest improvement overclocking the memory on the gpu but just play around with the settings app until you get a result that works for you.
Ive been mining on my old r7 250 to see how things works and cranking the memory up gave a nice ~ 20% or so improvement
I just ordered a hd 7950 that I found a pretty decent price that has good cooling on board. I'm a bit surprised you can profit as much as you can at this point and hope we can keep at it for a little while at least until everyone catches up and smashes the hash rate with gpu farms in the future. I play games and wanted a faster gpu so I figured this was a good opportunity to upgrade for free at the least and make some money if all goes well.