We only have a single white head with 2 channels and not the dual heads with 4 channels, but it's slow. We do not run the standard inkset so that could potentially help. You can also run the color on a 4 pass and come back and do the white with whatever you need for opacity at the slower speed.
We do 5 layers of white when doing back splash prints for kitchens etc. If we could do 8 layters in 11 minutes, we would... but on our machine it'd probably take an hour per panel and not worth the time.