Latency is about what you can use is real usage. How good it works when other part of your daw also requires resources, like IO, CPU, graphics etc. If your real life workload is flawless with 32 samples buffer it is as good as it can be. I can also use 32 samples on my fireface with firewire. But on real project with say about 20 tracks, 10 vsti’s and 10 audio recorded. Not a chance with 32 even with low sample rates as 44.1 kHz. I usually use 192 samples buffer, but is not perfect. Occasionally there are drop outs.
Just as a matter of practical use: Dont be too critical about latency! Most people tend to forget that in the real world relatively small distances between the source and the listener also create significant delay! My recommendation is: Trust your ears - as always. Some miliseconds latency (and thus delay in certain situations) are usually not audible or at least not different from natural phenomenons like the one I mentioned.
Don’t expect any dramatic differences on RME devices between thunderbolt and usb.
Their way of utilizing usb with dual firmware (PC and MAC) makes then very close.
Making low latency thunderbolt devices is a lot easier as it is essentially just PCIe on a wire.
However higher bandwidth does not automatically equal lower latency.