[[ Only if you have a Thunderbolt-capable Mini ]]
Nope. Not at all.
USB3 is as fast as Thunderbolt -- in most cases, you'll notice little difference or none.
USB3 is a "completely different animal" than was USB2. It now uses something called "UASP" (USB Attached SCSI Protocol). This relieves the computer's CPU of the processor cycles that were previously required in USB2.
USB3 with UASP seems to operate much like SCSI did on the old "Classic" Macs -- that is, it "works on its own" without robbing the computer of CPU time...
Prediction:
The next iteration of USB ("3.1"?) is going to leave thunderbolt in the dust, insofar as speeds are concerned. It may push thunderbolt aside, the same way that USB3 has pushed firewire "out of the picture"...