I currently have 2x4 GB Corsair Vengeance in my motherboard. Could I add 2x8 GB to the other two open slots? (I would have 24 GB total) If so, how will it affect the performance?
performance depends on what you do with your build.
You can putin more memory. You just need to look that you put same size memory to same memory channel. Two channel so smaller set to other and bigger other. Or something ^^ In gaming you will not get any performance boost with bigger memory. Video editing and something like that it will help.
Someone will have to check me on this, but you should be fine. All of your ram will run at the speed of the slowest pair.
Otherwise your performance shouldn't be affected. If you're video editing you should see an increase in performance. I don't think there will be any difference in gaming.