Block sleep - OFF or else it won't sleep!
Usually the issue with sleep is that on waking up from sleep, it crashes. So you would turn block sleep ON if you wanted to stop it sleeping and crashing on wake. However, turning on Block sleep in BIOS will also disable sleep in Windows. Rather than that, use a Mac app like Amphetamine which works quite well.
I can't think of any other settings specifically. I did turn off CFG with the BIOS hack, but not sure if this helped or not. On my other Dell XPS, I didn't touch CFG and they all sleep fine.