Hey guys,
I can now confirm a couple of things, for I succeeded in building my favored built:
Z97 Gryphon
GTX 970
Asus TB2 -Dual- Card
running El Capitan with Clover
the natively supported Broadcom BT/Wifi Chip
Essentially: Everything works. I can even couple an Apple Thunderbolt Display to the discrete Graphics with full GPU Output - nice!
Had to first activate the thunderbolt card with drivers on Windows and since then it runs like a charm. On windows, it's even hot-pluggable. For OS X, any TB device needs to be connected on startup, though.
Yes, the Asus TB DUAL Card is working with the Z97 Gryphon Mainboard, even though it says otherwise on the net and in the manual.
As for the setup with the discrete graphics: I connected the display port from Thunderbolt Card -> GTX 970 and Thunderbolt Card -> Thunderbolt Display, no adjustment needed. Sound's working, but so far I did not manage to achieve any of the other Thunderbolt Displays functionality. If anyone is aware of a workaround for this, it'd be much appreciated.
EDIT: Oddly, the Thunderbolt Display seems to -fully- work sometimes. I'd tried to figure out how and when and to see if it's reproducible - it is. If I first start into Windows 10 if an additional display (in my case attached via HDMI) and then proceed to reboot into OS X, every ATD functionality is working as intended and I don't need to keep the additional display on or plugged in. Of course, rebooting or shutting down the system creates to need to redo this procedure, so it is somewhat annoying if regular switching is in use. I'll add any further observation and/or solution as soon as they occur.