So we got a bug filed in Fedora 23 that the nvidia GPU wasn't stalling on resume from runtime poweroff. When we looked into it, we noticed the GPU wasn't powering off and the delay was from things going to hell after that.
I dug out the DSDT and noticed it has special Windows 10 handling code for the SB.PCI0.PEG.VID device. Adding acpi_osi="!Windows 2013" made things work again and everyone rejoiced and had battery life again.
Now what to do about this? I doubt this is restricted to the W541 laptop, I assume all Optimus laptops that support windows 10 will have this problem, so I'm not sure blacklisting on a case by case basis is going to help.
More information from investigation: On Win7/8 paths, the way to poweroff the GPU is to call an ACPI _DSM on the VID device that sets a magic bit, then the next time you go into D3 the _PS3 method gets called and powers the device down. This is to make up for the lack of D3cold in Windows at the time I suspect.
Now with Win10, the PCI0.PEG device has power resource methods, specifically PR3 which enables D3cold. However nouveau is bound to the VID device not the PEG device, so never attempts to power it down using those methods. I've hacked up the attached patch, and it does indeed power the device down, but I've no idea how this is meant to work in real world.
Dave.