HKSPete have you toyed idea of super-charger driven by variator? I think it would be better. Just to mention few good points, stepless RPM increase/decrease, less complicated, less wearing parts, maybe more favourable cost wise...
But my personal view to this topic is that compound turbos and similar setups are just obsolete and belongs to yesterday. With todays technology we have well-designed sequintial twin turbos and variable pitch turbos(dunno what it's called in english).