I run my UFX via USB. One of the first things I did was record all 30 channels at 192kHz for about an hour. The thing didn't even break a sweat, let alone use up even a fraction of the USB bandwidth at any point. I even wrote to an external USB hard drive utilizing the same buss. The latency is comparable to most thunderbolt interfaces as well. It is all about driver stability.
The only reason to upgrade to thunderbolt for audio is the need to operate more that 5-6 hundred channels at a time, to daisy chain data, video, and audio devices, or the simple convenience of using a single port. There is no audio data or "speed" performance advantage.
That being said, I am very impressed with the dynamic range of the new MOTU's. Preamps aside, you won't find converter quality with those numbers below Mytek for anywhere near the price. I have heard reports of the build quality to be cheap and "plastic-y", though. Like most modern hardware, it utilizes budget over-seas manufacturing. There is a trade-off for the price. I wouldn't take it on the road.