Old topic, but I am also looking for the same.
Looks like the P2000 can be as fast as the P4000 if you overclock.
Also have a look at this: https://docs.nvidia.com/video-technologies/video-codec-sdk/nvenc-application-note/index.html#nvenc-performance (Table 3. NVENC encoding performance)
The performance varies across GPU classes (e.g. Quadro, Tesla), and scales (almost) linearly with the clock speeds for each hardware.