"They claim their APU pushes out 3.2 times as many draw calls in DX12 versus the Core i5-4570R pushing out 1.69 times. All this tells me is how much more the APU gains in DX12. It doesn't tell me the absolute performance of either part. For all I know, the APU is pushing out 320K calls on DX12 versus 100K and the i5-4570 is pushing out 338K calls on DX12 versus 200K draw calls. Plus if I recall correctly, 3DMark's test pushes out draw calls until the frame rate lowers to a minimum target (30FPS? 60FPS? I forget)."
If you want to know absolutes there are plenty of sites that have done those benchmarks. Its more like the amd igp putting out between 2-5(number remembered off the top of my head) times as many draw calls as the intel igp in DX12. However, that doesnt mean that its going to be increasing that many times of FPS. Reducing draw call overhead will help a lot, especially going forward, however you'll probably see single digit growth in fps all other things being equal. Maybe 5-15% increase from the draw call overhead change. Tho certain cases will be much higher then that.
With all the intel IGP chips out there, i think its very likely all the main game engines will cross load the IGP as well as the GPU. Well see it in all the major games. DX12 doesnt care what brand the hardware is, it presents a compute unit to the system for use. Im definitly not familiar with the guts of DX12, however using a nvidia and amd graphics card together should be no harder then using a intel IGP + nvidia card, should be no harder then using a amd igp + amd gpu + nvidia gpu. Or any other combo, they are all just compute units.
That doesnt mean you can just add up the performance of everything and expect linear growth. Dont expect them to be able to extract 30fps(gpu1) + 25fps(gpu2 different brand) + 10fps(IGP) = 65fps(+116% of gpu1). I would expect to see something more like 30fps(gpu1) + 25fps(gpu2 different brand) + 10fps(IGP) = 45fps(+50%), and as long it doesnt add any studdering, im totally fine with that.