Do you need more ram ? make sure that you need it...
I don't recommend going with a kit with 1.65v, the maximum voltage for ivybridge is 1.5v, so if you want to add ram, get it with no more than 1.5v, and yes, you can do it with 2x8GB or 2x4GB as long as it's in the same channel(color dimm slots).
You don't need faster speeds or different timings, it will not increase the performance.
ok thanks but i meant if its ok to add the 2x8 kit with timing that is diffrent than the one i have