Onboard vs NICs

[PhiloEpisteme]

I posted recently about an upcoming build. I'm still deciding on hardware etc and feel no closer to selecting a board than I was a week ago. Nonetheless, I would like to try to clear up something regarding the difference between onboard controllers vs NICs.

What I am specifically curious about is whether, in high throughput environments, there is a real disadvantage to using the onboard interfaces vs the NICs assuming quality controllers.

For example, consider a situation where one has a 4-port board such as the Supermicro M11SDV-4C-LN4F which has 4x 1GbE with Intel I350-AM4 with a PCIe2.0 link per the ark.intel.com pages and pair that with an Intel I350-T4V2 4x 1GbE card which uses a PCIe2.1 link. Would one expect these onboard controllers to perform comparable to the NIC given that they are both quality controllers? Or will there be some performance penalty due to perhaps the onboard controllers making more use of the CPU, being slow to get data between the onboard controller and the PCIe bus to the NIC, or some other reason?

If there is no serious performance penalties (assuming one purchases a board with quality onboard controllers) then I will likely consider one of these 2-4 port boards with i350 controllers and add as many additional NICs as required to bring my port count up.