Not sure about the utilization, but it's impressive, that with just 12 cores 1440p 60fps @ slow preset was only giving 1,7% encoder overload.
Very interesting, that the second stream test was giving way more trouble (15,4% encoder overload) although this was just 1080p 60fps @slow preset.
Only difference, besides the lower resolution seems to be the Webcam (first stream = scene with capture card, second stream = scene with FullHD webcam footage.
So maybe the driver of the Webcam is causing high load, when using 1080p on the cam.
Either way...with only 8000kbit/s, fast movement in vegetaded areas will be very pixelated/blurry in 1440p 60fps and even 1080p 60fps (no matter if it's just medium preset or slow...slow preset won't give you a visible advantage over medium).