The memory and cores of graphics cards don't operate like a CPU. You don't have to worry about one 'bottlenecking' the other because it doesn't work like that. You just keep cranking it as high as you can simply put
Yes there are RAM timings for cards, but not like the normal timings you're used to and I highly suggest you don't even bother with them.
Download Nibitor 2.0
You can open your cards BIOS and play around with options in there, but yeah I'd only do that so you can look at whats in there, not to actually change anything