The GTX 1050 and 1060 families have one NVENC engine. The GTX 1070 and 1080 families have two NVENC engines. The GTX1030 uses a GP108 chip and I am not sure if it has any NVENC engine at all.
As I have mentioned before, these engines are capable of about 350 frames per second each of high quality 1080P H.264 and so the speed is dependant on the speed of the encoding streams feeding the engines. For 1080P high quality, it seems the optimal number of streams need is about 9 for two engines and 4 or 5 for one engine. That is based on the nVidia data on the Quadro cards that use the same chips as the GTX family and that are allowed to run unlimited numbers of encoding streams.
Licensing issues means that the non-Quadro cards are limited to two streams so that is the bottleneck. Potentially, you could get the similar NVENC speeds from a 1060 or a 1080. When you allow Vegas to use CUDA rendering, it uses it for the special effects in the editing, so it really does help if you have added a lot of effects. If not, CUDA rendering doesn't speed things up at all.
The GTX 1050 chip seems to only be capable of two HQ encoding streams but if the streams are efficient enough, it still could possibly perform almost as well as a GTX1080. It all comes down to the software feeding the cards.
So if you have a GTX1070/1080, you have a card with an encoding engine capable of 700 1080P High Quality H.264 frames a second, you are doing very well if you can feed it quick enough to get 200 frames per second.
If you are the proud owner of a Titan, then you have 3 NVENC engines but you can still only run 2 streams. I guess it is always handy to have a spare.
I don't have a fast modern CPU right now but I did find MediaCoder seems to be a fairly efficient transcoder that includes NVENC. You are meant to have to pay for GPU encoding, but NVENC worked fine on my free version. On my old system, it did seem to use the CPU processors more efficiently then Vegas. It is possibly a good way to get the real NVENC of a CPU/NVENC speed. It used NVENC 7.0 and the latest SDK is NVENC 8.0. You should have nVidia drivers 378.66 or newer (Windows) or 378.13 or newer (Linux).
http://www.mediacoderhq.com/download.htm