The prototype

The V-Boost system has worked well with the donated Vicor power blocks performing trouble free at all temperatures. thanks Vicor. The problem with the concept is that the Vicor power blocks cost over $2K and thats just for the components.
What did I learn:
MIMA manual control of the electric side of the hybrid drive is essential for getting the most out of any PHEV system.
I want to drive with the flow, and tend to use MIMA rather aggressively.
I can draw 16.5A of charge for my HV battery by drawing 65A from my lead booster monsters in the back.
I found that for my type of driving, 16.5A was not quite enough to keep up.
Other driving conditions and speeds may be happy with much less. Having the ability to tune the charge rate on the fly will be a feature of the Buck converter design.
As discussed any boost or reduction in voltage between the PHEV battery and the stock battery will come with an equivalent increase or decrease on their input current. Input watts = output watts - efficiency losses.
Boost
The biggest disadvantage of a low voltage to HV boost system like my V-boost and the Engineer system is the high currents required from the LV battery for reasonable currents in the output.High currents means lower efficiencies,less reliability and higher operating temperatures, and shorter battery life.

As a firm believer in recycling as the best way to save energy, so recycling the many prius packs into Insights for PHEV service seems like a way to kill two birds with one stone.
I want to recycle the Prius drive components as well:
EV Insight
Time will tell