In case you missed it, Stephen did provide some detail on the expected performance differences between the Stream 3 and what it replaces in the Pulsar and ZENith Mk3. I’ve copied that below for your reference.
You’ll find that the Stream 3, like the Pulsar does offer a performance improvement over the ZENith MK3. You may notice a further bump in performance too now with the introduction of RTK (real time kernel) too!
That said, you currently have the standalone PhoenixUSB, which will deliver better performance than the integrated PhoenixUSB Lite module you could fit in the Stream 3. However, the latter does offer a very neat one box solution.
It is definitely something worth exploring with your dealer once units become available. In your case, I would personally consider upgrading to the ZEN Next Gen paired with the PhoenixUSB Lite. With a trade-in of your ZENith Mk3 and PhoenixUSB, this could be a more attractive option overall and a larger improvement in performance for you. Food for thought!
"STREAM3
(versus ZENith Mk3 & PULSAR)
The STREAM3 shares a fair amount of DNA with the PULSAR since this proved to be a very well-received and effective platform. Consequently, this means STREAM3 is a very solid upgrade over a ZENith Mk3 in terms of sound, although existing PULSAR owners may find sound quality to be broadly similar. The same PSU with 300VA transformer and ARC6 active rectification module is present, as is the CX regulator and solid 10mm CNC-machined chassis. The STREAM3 has a TLC SSD rather than the pSLC of the PULSAR, but STREAM3 does have Real-Time Kernel support. Essentially, the STREAM3 could be thought of like a PULSAR that now has storage and music server capabilities (along with all the other generational benefits mentioned earlier in this post)."