Throughput isn't an issue during mixing for any protocol, only say recording simultaneous tracks. That's the biggest place this would matter from a bandwidth perspective but in my experience a non-issue until you are recording > upwards of 30 incoming tracks simultaneously.
The biggest issue with USB and latency or bandwidth is the manufacturer, some use class compliant or substandard USB driver implementations and those can bite you. However, something like RME or others of that quality, this will not be an issue because they write their own USB driver stack. I have an RME UFX and easily recorded 24 live tracks on a MS Surface Pro 2 over USB and to a 32GB micro SD for that matter.
As far as low latency settings in general, there is no need for lower buffer sizes/low latency when mixing except:
1. You have/need sample accurate automation during mixing.
2. You are recording something like an amp SIM/MIDI whathaveyou.
3. Whichever use case I missed.
When those are not the case, just set the buffers higher so as to not waste CPU for no reason. All that said, I'm sure there are requirements I didn't glean from your initial post so feel free to fill in the blanks - the short answer is that if the SC manufacturer did a great job with the drivers, even USB2 should be just dandy in the vast majority of situations.