I have used both a HiFace with my Theta Gen Va (balanced, 96K) and an ART Legato with a Theta Casablanca II (Xtreme DACs). For RBCD, the Legato beats the HiFace with reduced haze, glare, and sibilance, giving me the most liquid sound I have ever had. However, the HiFace sounds very nice, especially so with a 5M BNC cable. It might sound better in other systems, depending on the effect that my PC's power and grounds have on it. The Legato uses its own power and achieves galvanic isolation. I can get the HiFace closer to the Legato with a Monarchy DIP, but the Legato still wins.
Either of them beats my Theta Jade transport (which was connected using a 1M AES/EBU wire).
I think you're right on the money regarding the sound quality of the Gen Va. It was, and is, top-shelf. Effectively, the asynchronous USB interface performs a very similar function for Jurassic DACs that buffering/reclocking does in the Generation VIII.
Given that a Gen Va can be had for $1500 or less on Audiogon, pairing it with a top-flight asynchronous USB to SPDIF converter makes a compelling investment.
Since I'm using the BNC output of the USB interface(s), and a modest 5M BNC->BNC cable between the interface and the DAC(s), I am able to unload the 1M Siltech wires I used for RCA and AES/EBU connection for more than the cost of a USB->SPDIF interface.