This is an exciting project. Lancia had a great deal of success with this twin charging method. So did Alfa Romeo, although in that case they used a Roots supercharger blowing into another Roots supercharger.
A couple things, Roots superchargers are not inefficient at high RPM. At one time they were inefficient at high boost levels but that's becoming less and less true all the time. If you have any doubts, watch the video I post here showing a second gear pull. Notice the rate of acceleration does not drop off at high rpm. You should also notice the low discharge temps on the red digital gauge (outside air temp was about 65F). Adjustable boost is easy to set up on a Roots blower, although the boost control system on a turbocharger is somewhat more efficient.
In any case, I do think your design will give a best of both worlds situation and I am really excited about it.
Video:
YouTube - Supercharged Alfa Romeo GTV6
Greg,
hiperformancestore.com
P.S.
If you need crank pulleys to drive that supercharger, I have them in stock.