Gary,
I agree with your understanding that on a turbo car - the less the backpressure, the better... period. I think that a system like this could benefit you a lot. I know you're just doing it for the sound, but there's a potential for at least some performance improvement as well. I would not expect that on a normally aspirated car though.
I know Boggtown did the electric valves on his car. Personally I would lean towards vacuum operated valves though. With the electric valves you've either got them open, closed, or dink around with setting them partially open somewhere inbetween. I would think that with a vacuum operated valve though you could simply tie it to the vacuum side of the engine (intake) and let that control the valves. As your car generated more vacuum (higher revs) the valves would automatically open more, giving you more sound. When you came to a stop the valves would slam shut and the car would be quiet. You could still wire a solenoid in there to keep the valves closed if you wanted too based on an electric switch in the cabin. You could also put a manual boost controller (MBC) in-line and set it so that the valves only see vacuum and open after a certain vacuum level is reached (say above 3000 rpm, so there is no resonance). An MBC turned around 180 degrees is simply a vacuum control valve rather than a pressure control valve.
I see a huge number of advantages to going with a vacuum controlled valve as opposed to an electric valve. But maybe that is just my perspective. You see back when I owned a Subaru WRX I sold "Bristol Boost" control systems for their turbo (if you google this you can find my posts on the WRX boards regarding this mod). This system used solenoid valves and two MBCs to control a dual stage boost pressure system. It may seem complicated, but it's relatively easy using parts I made or modified.
Looking at the PSE system with vacuum valves I've been kicking around the idea of using my Bristol Boost type of control system on these kinds of valves for controlling exhaust noise. The result would be more noise, controlled by you, with NO resonance since you control the exact kick-over of the system.
Right now I'm waiting on a Gemballa exhaust for my car, which I should receive in a couple weeks. Once I've got that on the car I'll see how it sounds and I may look into a source for these vacuum controlled valves a little more to maybe modify that exhaust.
Just throwing out some more food for thought and giving you a glimpse of what I've been considering.
Kirk