Maximizing FPS of ffmpeg encoding
There is a special NVIDIA API that allows you to encode to h264/h265 called NVENC. It's now integrated into ffmpeg so using it is quite easy.

But before buying an NVIDIA card you should do/know the following:


Check the NVENC hardware capabilities table from the NVIDIA VIDEO CODEC SDK APPLICATION NOTE - ENCODER cause h265 support is limited in Maxwell V2 card you've mentioned. You can also find average encoding speed (fps) there.
There is an artificial limit of two for parallel encoding processes for GeForce cards (check the previous link for more details). If you want more (actually around 30 simulanious encoding streams) you should buy Quadro or, according to this beautiful answer, one can hack GeForce (hardware in fact is the same as in Quadro) which is, of course, illegal.
With any hardware encoder you'll never get the same quality/size as you can get from x264.
Also there is Intel QuickSync hardware encoder integrated in some of Intel's processors (not in your Xeon unfortunately) which I know nothing about, so you have to research this part yourself.