Hi Mark,
Here is the philosophy behind this decision:
We are generally avoiding Velocity XFade for alle articulations with only one layer, also because 2 identical layers would have to be triggered, which doubles the voice count - a considerable fact with our multi-mic libraries.
Psychoacoustics is a big topic for all libraries that were recorded in a big hall like our Vienna Synchron Stage (and that room-sound is an integral part of the sound). Changing the perceived volume of an articulation isn't something we'd suggest to do, which is why the difference between Velocity 1 and 127 is only 3 dB for those articulations.
In our eyes, this is also a question of workflow, as you can generate any kind of dynamic change by using the Expression Controller, which is the main task of this controller anyway.
"Timbre Adjust" will be an even better option at this point, with it's very natural dynamic changes.
I believe that you could achieve the results you are after by using 2 MIDI Channels, with the sfz on one channel, triggering Timbre Adjust with your player Velocity. Definitely worth a try.
Best,
Paul