My ZED mixer is connected with onboard FW (!), I’m running at 128 buffers with 3 ms latency. For mixing, tracking, everything. I recently added the insanely discounted ADA8200 for 8 additional in/outputs, all over FW, that’s 26 (!) channels total.
YMMV I guess… 
But when I built the machine, the OS has 1. never seen the onboard sound or ethernet interfaces, since I disabled all that before installing Windows. Plus, 2. the FW is using the same IRQ as the WiFi card, so I disable the WiFi before using Cubase. Works like a charm!
Cheers,
Benji