Just an update - I have picked up a PCI-E NIC to get this thing up and running just to see if it even recognizes the AR8151. It seems the correct driver is used and the controller is recognized:
0000:03:00.0 Class 0200: Device 1969:1083 (rev c0)
Subsystem: Device 1458:e000
Flags: bus master, fast devsel, latency 0, IRQ 16
Memory at fbcc0000 (64-bit, non-prefetchable)
I/O ports at df00
Capabilities:  Power Management version 3
Capabilities:  MSI: Enable- Count=1/1 Maskable- 64bit+
Capabilities:  Express Endpoint, MSI 00
Capabilities: [6c] Vital Product Data
Capabilities:  Advanced Error Reporting
Capabilities:  Device Serial Number ff-02-b3-f8-18-d6-c7-ff
Kernel driver in use: atl1c
But the interface doesn't show up with an ifconfig -a, nor does it show up in DSM.
Any other diagnostics that I could run to find out why DSM just won't pick up the NIC?