Okay I did some tests using Virtu MVP to force OBS to use either the HD4000 or GTX670.
Keep in mind, OBS is not officially supported by Virtu MVP and It's really tricky to make it work. I used GPU-Z to verify that OBS was indeed using the HD4000 and not the GTX670. There still may have been bugs.
My Computer specs:
i7-3770K @ 4.2GHz
GTX670/HD4000 GPU
16GB RAM
Test Method:
I ran a scene from The Avengers with lots of explosions and high contrast for about 2 minutes.
The VLC player had about 5% CPU utilization, it was the most efficient way I could get high motion scenes without much CPU usage.
So, can you run slow/medium settings? Let's check the results:
Code:
GTX670:
1080p@30fps@slow -> NO
1080p@30fps@medium -> NO
720p@60fps@slow -> NO
720p@60fps@medium -> YES
HD4000
1080p@30fps@slow -> NO
1080p@30fps@medium -> NO
720p@60fps@slow -> NO
720p@60fps@medium -> YES
It didn't matter very much which GPU I used, as the CPU was always the most limiting factor.
Now, there were differences. For example a frame at 720p@60fps@medium on the GTX670 was done in about 4ms, while on the HD4000 it needed around 8ms, which is still far below the maximum 16ms threshold.
From this I conclude that 1080p is not viable at slower presets, the 720p@60fps@medium looks great and offers a much better experience due to higher framerate(which is of course only my opinion).
I hope this helps, as it was A LOT of work.