You asked about timing and no one responded. We have a turbocharged slant six that is slated to run 20 pounds of boost, eventually, but the waste gate is set for 10 pounds right now, as we are still in the beginning phases of testing.
All the information I could find from people who have successfully made somewhere around 500 horsepower (and, some less) out of these motors (/6s) run 18 degrees total advance when under boost.
Flame fronts in boosted applications travel across the combustion chamber much faster than they do in naturally-aspirated engines... so, they don't need as much spark advance while boost is present.
You could use the vacuum diaphragm to increase the amount of advance for driveability and economy, when no boost is present, I'd imagine.