In a correctly configured setup (and in standard macs), iGPU does indeed not appear in luxmark: it shall be configured as connectorless and is then used for video encoding/decoding acceleration, while the 560 is used for opencl acceleration.
If iGPU appears in Luxmark, then some apps such as...
I think it is, and I would say it depends on the task that is assigned (decoding/encoding/which quality/which resolution).
On my side, I can see the iGPU frequency varying from a few Mhz while decoding a youtube video to 1.15 Ghz while exporting a 4K FCPX timeline
By vanilla, I mean only original kexts in L/E and S/L/E, and no configuration change outside of clover.
From what I understand, shikigva=1 should not be needed if you do not modify AppleGVA.framework to enforce offline rendering.
Quicksync is not for rendering, it is for transcoding (h264/h265).
There is no transcoding in BruceX, only opencl rendering through your 580.
Exporting a h264 video with quicksync is a magnitude faster than without.
Quicksync accelerates video a bit everywhere in macos: it decodes video in...
7-8 seconds for BruceX is typically when the timeline is pre-rendered. I would say that even 13 seconds is too fast for a 580. It should rather be around 17 seconds. You might have some rendered files already in your project. Deactivate background rendering, quit FCP and erase rendered files...
It seems acceleration works now ok in 10.13.2 beta 0. I had even +10-15% opencl performance boot for my Vega 64.
Still, sleep/wake does not work anymore, and fan spins very fast to keep the GPU cool, and is then quite loud... reverted to 10.13.1.
Nice 15% opencl performance increase with Vega in 10.13.2 beta 0 (https://browser.geekbench.com/v4/compute/1325116 -> https://browser.geekbench.com/v4/compute/1326514), but fan spin fast and loud, and Lilu panics now, so no Whatevergreen, and hazardous sleep/wake.