Analogy...
You have a car that tops out at 100mph:
You can put that car on a two-lane highway... or an eight-lane Freeway...
It's still going to top out at 100mph.
Adding additional (unused) bandwidth will not increase performance.
 
The advantage of Thunderbolt (PCIe level performance under ideal circumstances) is that you can set the ASIO buffer size down to 32-samples... some even go down to 16-samples.
This yields sub 3ms total round-trip latency. 
That's the only advantage vs. the best USB-2 audio interfaces.
Going Thunderbolt (or PCIe) with the audio interface will not yield greater processing power from a given machine.