I believe you're talking about hardware rendering option Magix AVC codec that uses NVENC hardware encoder on the Nvidia graphics card. The rendering speed would likely be very similar with the GTX 10** series cards. It's not really the graphics card rendering it's an independent module that's the same on that line of cards.
You will see improvments speed wise when it comes to filters/plugins that use GPU processing, that is utalises the actual GPU & not the nvenc asic . Or you should. I would hope you do. It's more difficult to predict with vegas as it's processing engine does not take full advantage of GPU. My GTX 1070 will use up to 80% of GPU power with video processing on Adobe Premiere, but Vegas uses less than 40%
I would expect some people here have benchmarked nvidia cards with vegas & could give you a clearer answer,