The high horsepower Chevelles of the day, the LS5 and LS6 cars all came from the factory using manifold vacuum. The LS6 was 11.25:1.
What people seem to miss is, changing to MVA really doesn't change much. I have the SAME initial timing with MVA with the can unhooked than I did using ported vacuum. The only change is WHEN the can is activated.