RAM bundled together in the same kit has been tested to run together at the rated speeds. When you buy RAM from multiple kits, even if they are identical kits, the ram will often not perform as well together. It is possible that your motherboard will derate the RAM from 3000 MHz to something like 1600 MHz (assuming you enabled XMP in your BIOS anyway to get the Corsair Vengeance RAM up to the 3000 MHz advertised speed). It is even possible that the two sets of RAM will not be stable together at all. If you already purchased the RAM, try it out. If you never enabled the XMP RAM settings in your BIOS, then it's possible the two sets will just run together at that speed. If you purchased identical kits, you can try and enable the XMP and see if they happen to work together at higher speeds. If not, you can decide if you want 16 GB of high speed RAM or 32 GB of low speed RAM.
To answer your question of if 32 GB will make your system run faster than 16 GB in general ... probably not. If you are doing a lot of video editing, or running RAM disks, or running multiple Virtual Machines, you would benefit from the larger amount of RAM. Otherwise, 16 should cut it. Especially if you haven't purchased the RAM yet and you haven't enabled XMP, you're better off just getting the RAM you have to run at a much higher speed than it runs by default.
You will get a small performance bump running your RAM at 3000MHz - not too much but a few FPS on your games. If you add 16 GB more ram, you probably won't notice at all, unless you something that is extremely memory intensive.