This is from http://www.sdsefi.com/
"In severe cases of retarded timing, the mixture is still burning when the exhaust valve opens. Because turbochargers are driven by the energy in the exhaust stream, high EGTs caused by retarded timing produce so much energy at the turbine that even a fully open wastegate cannot control the boost pressure. All in all, retarded timing is counterproductive to producing an efficient, durable, powerful engine."
So at the retarded map with a very free exhaust the wastegate cant flow enough. I had 20psi with stock maps and went down to 7psi with a advanced ignition map. nothing else was changed
If you did have a larger wastegate that would solve the overboost problem as would having more exhaust back pressure.